Are assassins evil?

[Emmerask]

To me, there is not even close to a definite difference between a Good guy killing the BBEG in combat after the quest and him sneaking up via stealth and "murdering" the BBEG to end his reign of terror. In the end the BBEG dies both ways, and each death is justified by the BBEG and ending his actions. The difference between a fighter and an assassin is method. It shouldn't have to be a difference of morality.

The means to an end are actually very very important if you want to consider an action overall evil or good.

I want to create world piece, a good goal isn´t it?
therefore I kill every human being on earth to achieve it

This is one of the most extreme examples but still to illustrate that means are of importance it is valid

As for the example of the bbeg, it could very well be that he sees the errors of his way when confronted with the heroes and if spared by the heroes after foiling his plans atones and becomes one of the major forces of good in the universe...
an unlikely scenario but still a possibility denied by the sneaking murderer.