Guild Wars 2

Why are thiefs able to finish people off and help downed people while invisible?

Everyone can. Just the thief and mesmer have skills to do it. Other folks can if they’re stealthed by an ally, or if you pick up a feather.

because stealth is a buff thieves have and other classes´ buffs do not disappear while engaging in these activities either.

stability stomp>stealth stomp

cry me ar iver

Pretty much what Wolfe said.

You can still be damaged and interrupted while in stealth…and it only lasts for 3 seconds unless traited for longer.

So take your qq elsewhere.

Because the thief was clever, and you didn’t do anything. Do something next time.