The current Assassination objective requires that the victim be dead by the time the round ends. It doesn't matter how many times they'd been killed during the round, they just have to be dead the moment the round ends. This tends to lead to two situations, one where the antag rushes down the victim at the last minute so there's no time for revival, and the other being to make the victim unrevivable.
There's a very limited number of ways that a body can be kept unrevivable, which leads to nearly all assassinations going down the same path of ambushing or rushing down the target and then making a break for it with the body before anyone can respond. There's very little, if any room at all for creative means of killing people, since anything that doesn't guarantee their body being unrevivable ends up being wasted time on the antag's part.
Antags are supposed to be a part of the storytelling for the server. They're meant to add tension, drama, and excitement. However, the goal of making sure someone can't continue to participate in a round goes against that philosophy. No longer is it fear that you may be attacked, it's now fear that you won't be able to play the game. It's a reason people late join to avoid being selected as antag targets, or overprepare themselves in case they or someone else is attacked. Granted, an antag doesn't need to green text, but doing anything outside of their goals requires admin permission. but the greentext still encourages antags to go with a method that is safe and most likely to guarantee the victim stays dead, permakilling.
Removing the 'dead at the end of the round' requirement I believe would alleviate a lot of these issues. Requiring the target to only die once would give the antags freedom to get creative with how they kill the person. There are so many cool and interesting traitor items and methods of killing that go unused because they don't give the opportunity to make sure the victim can't be revived. In addition to giving the antags more freedom, it would also mean victims don't have to metagame or valid hunt to make sure they will be able to keep playing the game. Many people say the best interactions with antags are the ones where they actually get to interact with the antags.
If there's worry about this change making things too easy for antags, there could be ways of making sure the kill was at the hands of the antagonist. Some servers have the antagonist put a calling card on the body of the victim as a mark of a successful kill. Other methods could include taking a part of the victims body as a trophy or branding them. It would also create potential for more interesting storytelling or interactions with the victim, as they may be the target of multiple antags, and each antag would get their chance at them. Overall, I feel like no longer requiring assassination to be permenant would be more enjoyable for both antags and victims, plus opening up the opportunities for more interesting interactions between the two parties.