Thinking of a support killer is like thinking of a killer doctor.
The idea is really contradictory but you can play around with it a lot.
I would suggest 3 different branches for the Assassin, one specialized in Poison and aoes, another specialized in the assassination itself through weapons and stabbing and finally a support which does not create objects or heals but rather steals them and applies them to their allies.
For example, the enemy healer is placing buffs on his allies, but the helaer assassin takes care of removing them and spreading them to his allies, canceling any type of buff applicable for 2 minutes to the person who stole them.
In turn, this can place a debuff which instead of the heler healing his ally or tank, the Assassin creates an anti shield on the enemy which absorbs the healing and explodes healing nearby allies in an area for a percentage of maximum healing.
Then if we are talking about implementing healing with the assassin, I recommend that it be through Darts, Darts or rather special arrows, which do not heal instantly but rather over time.
After all, a serum is released and affects the body in a given time.
So it could very well be a healer that sneaks in the dark, steals enemy buffs and enemy healing and can be in back rows healing allies in long times.
The DPS in an ASSASSIN is obviously necessary because the idea of an ASSASSIN is that it ASSASSINATE, but in turn it can not only kill the body but also the spirit of its enemies when healing and seeing that their healers or tanks can't protect them, decimate the enemy camp bringing hopelessness and overwhelm.