Dragonball has magical wish devices that undo death. That allows a lot of leeway when it comes to forgiving a badguy who has seen the error of their ways, or been inspired by the hero's awesome.
Even then, Goku didn't forgive Tao Pai Pai, Piccollo Daimo, Cell or Buu, and he killed all of them. (Well, Gohan got Cell.) He kept trying to convince Freeza to see the error of his ways, thnking that knowledge of a superior opponent could change him, and gave him five or six chances to give up, but ultimatley finished Freeza off. (That Freeza managed to survive is besides the point, Goku still killed him, and would have done it again on Earth if Trunks hadn't gotten there first.) And Goku killed tons of other guys if you wanna count the movies and GT. (and no one does.)
When you have an unrepentant evil, that evil has to be put down for good.
But when the badguy has DONE bad, but desires to not do it anymore because a better path has been made available to them, then forgivenes is okay, as is the case with Vegeta, Killua, Robin, Hiei, Kenshin, and Gaara.
Sasuke… has just uhm... gone plain nuts and blatantly declared that he desires lots and lots of murder or innocents, and he's incredibly excited by the prospect, not at all repentant about what he's done. He needs to be put down.
If you absolutely must bring up Vegeta's Maijin phase again, he was under the control of another badguy at the time, and under an extremely limited timeframe to fight Goku again one last time. ANd even then Vegeta was managing to keep his evils restrained to "I want to fight Goku". And two seconds after he fought Goku? He went to go make amends by dying to destroy the thing he'd unleashed, and protecting his son, AND his "enemy's" son. He knew full well the dragonballs could and would restore anyone he killed, and they wouldn't suffer or even remember the experiance. It was only an action to force Goku to fight him, something he was SUPPOSED to get at the tournament that he was now being denied. He DIDN'T actually attack or hit anyone he actually knew at that point.
Its possible Sasuke is under Madara's control, in which case… its being handled very poorly because that point hasn't been made clear. This is all being presented as Sasuke's actual opinion.