Should Bioware remove sync kills?

No, but the hydras melee and the destroyers death field (i guess its a type of melee) should be sync kills. No reason why the kett should get two and everyone else gets nothing.

Hydras do have a sync kill; if it swings horizontally than it's a melee. If it extends its right arm into a bludgeon and swing vertically downward and hits you it will initiation a sync kill; once hit it will then lift you up with its cannon and smash you down with its right arm(in its normal form). Enemies with a sync kill(Hydra, Fiend, Ascendant) do telegraph the ability(unless lag is present).
I remember playing an Apex mission against Remnant with the glass jaw modifier(+1000% enemy melee damage) on firebase sandstorm while on silver difficulty. I was taking cover in the lower left room to avoid observer lasers, then I tried to exit the room through the backdoor only to have a Destroyer block the doorway entirely and immediately use its death field thus dropping me in a second. At the time I was playing a human Soldier with 750 health and about 380 shields, then the field brought that to zero.