we can not tolerate these killings

[Celefin]

40. Removal of Assad and regime change has been explicitly ruled out as objective

So no, that most certainly isn't the objective.
Taking out the chemical weapons isn't either as it can't be achieved this way.
Stopping the killing isn't either, as it won't and will probably only increase it.
'Punishment' is an incredibly stupid argument and almost certainly not the objective, though one might never know. Those 'punished' won't be the ones responsible for the CW use.

Here's a speculative thought on a possible objective:
Taking out Syrian air defenses and radar to give the Israeli airforce hassle-free airspace to strike if they were to deem it necessary.
That might be a viable military objective although it is, as I said, purely speculative.