5e has plenty of rules for combat, but relies on rulings outside combat

I googled a list of medieval assassinations. In summary, they boil down to stabbing someone unexpectedly.

5e actually doesn't have rules for this. It has plenty of rules for combat - which is when aware, prepared, and actively resisting people fight each other. But otherwise nothing for stabbing someone in the back.

Instant death

In my games I run with instant death. If the party sneak up on a band of sleeping goblins and slit their throats, they die, no rolls needed on the attacks. If the rival guild manage to catch the player asleep, same thing.

Here's the kicker, the difficulty in assassinations is everything leading up to stabbing someone in the back. Figuring out where they sleep, when they sleep, how to get in there, then actually sneaking into the room. The final stab is easy.

Make sure to telegraph

This is great gameplay when it's PCs doing the assassinations, but when the PCs are the targets it's a lot less interesting. If the party don't know something, it functionally doesn't exist. If a PC wakes up dead one morning it doesn't make a difference to them whether the attempt was planned by the guild for months.

There needs to be clues so the party can react. Some people will say you need so many clues that the party definitely get the hint. I disagree. As you said, it's ok for the PC to die. So when the PC does die, you want the party to at least be able to look back and say "ah, this and that were clues that this was coming".

5e has plenty of rules for combat, but relies on rulings outside combat

I googled a list of medieval assassinations. In summary, they boil down to stabbing someone unexpectedly.

5e actually doesn't have rules for this. It has plenty of rules for combat - which is when aware, prepared, and actively resisting people fight each other. But otherwise nothing for stabbing someone in the back.

Instant death

In my games I run with instant death. If the party sneak up on a band of sleeping goblins and slit their throats, they die, no rolls needed on the attacks. If the rival guild manage to catch the player asleep, same thing.

Preparation is the key to successful assassination

The difficulty in assassinations is everything leading up to stabbing someone in the back. Figuring out where they sleep, when they sleep, how to get in there, then actually sneaking into the room. The final stab is easy.

Make sure to telegraph

This is great gameplay when it's PCs doing the assassinations, but when the PCs are the targets it's a lot less interesting. If the party don't know something, it functionally doesn't exist. If a PC wakes up dead one morning it doesn't make a difference to them whether the attempt was planned by the guild for months.

There needs to be clues so the party can react. Some people will say you need so many clues that the party definitely get the hint. I disagree. As you said, it's ok for the PC to die. So when the PC does die, you want the party to at least be able to look back and say "ah, this and that were clues that this was coming".