First, make sure your players are really on-board
If your players are telling you that their characters would kill that character on sight, they are telling you one of two things:
- The objections come from the character. "I am playing a character who would, due to their personality, kill the bug-ghoul on sight. I, as a player, want to preserve the integrity of my character's personality, but I have no particular preference for killing the bug-ghoul."
- The objections come from the player: "I, as a player, do not want to play a game where the bug-ghoul is a member of the party. The actions of my character are my only agency to affect the game world, so I will prevent this by having my character kill the bug ghoul if you try to force them into the party."
The solution is very different depending on where the objections are coming from, so the first thing to do is to talk to your players and find out what's going on.
Why? (A personal anecdote)
I have a story which is related to this situation. It's not quite the same, but it's similar-enough that the core lesson applies.
This was with an inexperienced DM; there was no session zero, and we were all friends (of the DM) going into the game. Character creation was done independently, without knowing what anyone else was going to be playing.
I brought a paladin knight errant character, and another player brought a barbarian who believed in might-makes-right as a moral principle ("The weak have no rights."). On the surface level, these characters are oil-and-water, but I didn't raise any objections. After all, I am a long-time member of RPG stack exchange, well-aware of "My Guy" syndrome, and I knew that I could always choose to have my paladin choose not to quarrel with the barbarian.
Our differences ran deeper than that, though: I wanted to play a heroic story, where we go around saving towns and making the world a better place, while the other player wanted to play an us-against-the-world story, where we go around conquering towns and demanding tribute. The DM was trying to split the difference by giving us towns to save and then letting the Barbarian conquer them and demand tribute over my character's strongly-worded objections.
That lasted about 5 sessions before I decided that a Bad Game is worse than No Game and quit.
Getting clarity from your players
I recommend the following approach:
- Talk to each player individually.
- Tell them that you're considering two options - a group brainstorming session to figure out a way to introduce the bug-ghoul to the party OR the bug-ghoul's player makes a different character.
- Do not ask them to pick one or vote - just try to get a sense for how they feel about each option. Well-meaning players may "vote" for the more cooperative option (in this case, the group brainstorming session), even if they have a preference for the other.
- If no player expresses a strong preference for the bug-ghoul's player making a different character, go ahead with the brainstorming session. Otherwise, ask the bug ghoul's player to make a new character.
Other answers cover how to introduce the bug ghoul
I won't re-tread what other answers have said. If your players are on board with it, refer to other answers for the details of how to run the brainstorming session, and what sorts of ideas to start with. Work with them to find a solution where their characters can choose to accept the bug ghoul into the party.
If players don't like it, make the bug ghoul player's play something else
If you have players who have a preference against the bug ghoul, you should own those objections yourself. Tell the bug ghoul's player something to the effect of, "I've been thinking more about it, and I don't think that the character you're proposing fits with the game we're playing."
Then help them roll up a new character and guide them toward options that do fit with the game you're playing.
If you're not sure, go with a different character
If you can't get a clear signal from your players about whether the objection to the bug ghoul is from the character or from the player (even if the lack of signal is for perfectly innocent reasons like not getting a chance to talk to them one-on-one), you should assume that they come from the player.
I recommend this because the consequences of wrongly forcing the character into the party are much more severe than wrongly forcing the new player to come up with a new character.