This life stealing skill is absolutely worthless and such a waste of stats. It’s why his damage is so low (which, as Revelate noted, makes the healing weak too). That’s the same skill that Renfeld, a 3* purple has, and he too, is so very sub-par to heroes like Balthazar, Bane, Nashgar, etc.
While it’s fine for a 3* to have a garbage effect on his skill, on a 5* that requires such investment to level up and ascend it really sucks.
Maybe if he healed 100% of his damage dealt. Maybe. Though even then, I think he’d be a weak character since when healing is needed, healers are taken.
Now, how about if his heal actually affected the entire party? A fast attacker that heals the entire party for say, 25% (or whatever is reasonable, that’s a random number)? That could be something to consider and it would let him keep his identity as a damaging character with healing attached.
Perhaps it would also give a 5* alternative to Boldtusk. Instead of buffing attack, he’d deal damage and similarly heal the party.