Which servant should i roll ?

Yeah it is a tough choice, I agree. It seems to me, from seeing your previous responses, that you have a slight preference for Ishtar and Musashi over King Hassan. If that's the case, go for them.

However, to answer your question in more detail, you're right that King Hassan's role as a damage dealer is a bit weird. The 0.9 assassin damage modifier does hurt his damage a little, but not as much as you might think: for example, archers have a modifier of 0.95, and it really doesn't affect them too much. Gilgamesh is considered one of the best damage dealers regardless. The servant just needs a good enough kit to make it so their modifier doesn't impact them much.

Also, when looking at damage output, people tend to compare servants with others in their own class and among assassins, King Hassan's damage is one of the best (Jack is arguably as good) - when paired with Merlin though, no assassin can match his damage output as he is the only buster assassin in the game.

That said, you're right that when you look at things more generally, his damage output won't be as high as damage options in other classes, like Musashi. Also, you're right that his damage isn't his only strong point, and that his skills are good too. They are arguably why most people like him (other than his design). They make him straight up the hardiest assassin in the game. His guts is the best in the game and gives him just under half his health back, and protection of faith EX gives him a TON of buffs with no drawback. Debuffing is common in tougher content, so the debuff resist from this skill is actually pretty handy. Evening bell also comes with a 50% boost to his buster card damage, so he can hit very hard when that skill is up.

If I'm not considering your servant lineup and just looked at things extremely generally, Musashi has a higher damage cap than King Hassan, but King Hassan can generally survive for longer. One point of note though is that Musashi is better equipped to deal with challenge quests because her skills counter a lot of gimmicks that pop up in them. Musashi is probably the more useful option in slightly more cases, but King Hassan is more unique - there is not another buster assassin in this game as far as I know.

As for King Hassan's weaknesses, other than the 0.9 damage modifier, his kit with 3 buster cards also limits him more than helps him imo. His base NP and star gen are both excellent, but because his deck is so full of busters his overall NP gen and star gen is inconsistent as a result. And you're right, his instant death really doesn't amount to much. It's not that his instant death is worse than anyone else's, in fact his is one of the better ones. Its problem is that it's limited to his NP, which you would really only be using on bosses, which all have extremely high resistance to instant death. It'd work pretty well on weaker enemies, but honestly there's not much point to that.

In the end it's really up to you, but like I said, judging by your response to the previous poster, I get the impression you slightly favour Ishtar and Musashi, and that's fine. The most common advice anyone ever gives is to roll for who you like most, regardless of their gameplay value. Considering how good the servants are is really just secondary, since this game can be beaten with pretty much anything.

EDIT: Okay OP sorry this is such a huge wall of text lol didn't realise it'd gotten so big.
TL;DR: Just go for who you like best. No need to overthink it :)