If god is omnipotent he could just snip his fingers and will satan out of existence.

He will.

Something like the great flood is also a horrible evil when, god being omnipotent, he could just snip his fingers and make people not sinners

The flood was God punishing and destroying evil. People had free will

If he’s not actually omnipotent, sure, then a lot of things change, though overall I wouldn’t describe old testament god in particular as kind anyway. Also people seem to really insist on the omnipotence.

The Old Testament God is the same as the one of the New Testament.