I think this is a really complex question, and depends on each situation. While I'm inclined to say a resounding blanket 'yes', it's simply not that simple: for instance, I don't think I (or any other private citizen) have a right to do your third suggestion. Violence should be reserved for the state - and if you go and kill someone just because you know they're a terrorist, you're a vigilante, which is never a good thing. If, however, after a fair trial the state decides to sentence a terrorist to death (not that a trial of a terrorist is ever going to be completely fair or unbiased, but we're operating in a utopia in this hypothetical), then that is fine. Your duty as a citizen is simply to inform the authorities, maybe detain the individual. Killing is not the answer. I firmly believe that you cannot just kill anyone, even if they're a 'bad' person. The only exception would be self-defense, and even then, I think you should aim to avoid killing the person. Everyone has the right to life, even bad people.

Your other hypotheticals, however: I think if you don't pay your taxes, that's not necessarily "evil". Honestly, everyone's ultimately wanting to pay less taxes. There's all these scandals in the news about Starbucks and Google doing offshore business to avoid paying taxes... It's human nature to want to pay less tax. You're still paying VAT and other 'automatic' taxes, so if you sell some objects on the side and give the money to charity, I don't think you're doing anything super questionable, from a moral standpoint. I suppose it could be argued that the money would've gone to a 'worthy' cause if you had paid the taxes, but it did anyway? So, yes. I don't think that not paying your taxes is altogether that evil.

@Tom_The_Builder - you seem to have a different opinion on this, and view not paying taxes as "stealing". I think that's a very utopic worldview. Not all your taxes are going towards old people and children. A lot of your taxes are going towards upkeep of presidential palaces, and government jets and the fleet of government cars, and your country's military upgrades, and the roads of other cities, and the garbage disposal system etc etc etc. Yes, a portion of your taxes goes towards paying pensions and schools. But that is just a portion. The government does a lot of things. And if you don't declare 100% of your necessary taxes, it doesn't mean you stole from a needy child.

Finally the situation with the innocent man in court... I don't know if it's ever a good idea to perjure yourself. Is this really the ONLY way to prove that man's innocence? And how would you know that he really is innocent, if there's no indications aside from your lies? So potentially that's not the best plan. If, however, you perjure yourself in court to get a friend off for a minor infringement (we're not talking murder/rape/assault here, we're talking, maybe drug possession), I think that's not necessarily a bad thing. Prison is a really terrible place, and you'd never want someone to end up there for something minor. And while it's well and good to say that the justice system is bound to do its thing, the justice system slips up. A lot. So.

I really think every situation is different. And, yes, sometimes it's worth doing something questionable (also, keep in mind that 'evil' is a very big word - some things which may seem 'evil' to you or Tom_The_Builder are not evil to me) in order to get a reward, or avoid a bigger injustice in the future (such as a pothead going to prison and getting prison raped and becoming a gangster).

This is a very tricky ethical dilemma. I would like to speak about about not so tiny but definitely evil act for the matter of survival. You may have heard this story or have seen a movie "Life of Pi" loosely based on the events of that famous murder and trial.
So on July 25th 1884 Captain Dudley killed and later began to eat Richard Parker, his sailor. He shared Parker’s body with two others, Edwin Stephens and Edmund Brooks. All terrible as it appears this case became very controversial and there were many who defended the right of these 3 people to feed on poor boy.

So lets go back in time. 20 days earlier four men had been in the middle of the Atlantic when their yacht began to sink. They clambered into a life-boat with only two tins of turnips and no fresh water at all. They were consistently failing to gather any fresh water for 20 days, and though everyone knew how sea water could kill you, the youngest boy couldn't resist and became really really sick from drinking sea water.

For several days he was drifting in and out of consciousness, while Captain Dudley and others decided there was only one solution that will help them to save their lives; one of them had to be sacrificed for food. He proposed they draw lots. But since the boy was unconscious and Brooks objected, they decided it has to be the boy. The reasoning was simple. The boy is going to die anyway, and he is the only one who doesn't have any family to feed. So that night Dudley and Stephens stabbed Parker in the throat with a penknife.

For four days, until they were finally rescued, Dudley, Stephens and Brooks fed off Parker’s carcass.

When they were back to England they were candid enough to tell the truth, hoping that the would be proved innocent because the murder was carried on out of necessity. But a court consisting of the top judges in the country found them guilty.

This story had made a huge impact in modern understanding of law and justice. Suppose you find yourself in a situation in which killing an innocent person is the only way to prevent many innocent people from dying. What’s the right thing to do? Killing a minority in order to save majority is a slippery way that can lead only to fascism and cast system.

Talking about life and death is an extreme example, but thats how we understand better that there is no place for a compromise in the ethic system. There are no big and small things.

Lilly, story you told is so terrible and I can agree when you say,” Talking about life and death is an extreme example, but that’s how we understand better that there is no place for a compromise in the ethic system. There are no big and small things.” However every day we take daily decisions and every day we weigh pros and cons and most people do it on the basis of good and bad. I even say more we do it on the basis “what brings us less bad and more good”. So every day we decided not only what is good or bad, but also how much it is good or how much it is bad and anyhow we take such decisions.
If talk about ethics… The 3rd situation described in the post is ethical: to kill or not to kill. The first and the second are not ethical for me. They are just ordinary cases from our lives and I am sure that everyone here has tried to avoid or avoid paying some taxes and we are not sending them for charity at all.
To perjure in court is also not so ethical. Wouldn’t you lie for your husband or children even if they are guilty?
We all human being and we turn the ethical laws as it is convenient for us.