If the ability doesn't say Attack, it doesn't count as an attack. So, if the Attack ability is replaced by a Kill ability, it's no longer an attack.
What if I consume an element to turn an attack action into a kill action?
So long as you consume the element and trigger only the kill effect, the word "instead" negates the attack, and it is no longer an attack.
If an ability allows me to "kill" an enemy, is that considered an attack?
No, unless the ability says "attack," it is not an attack.