Hamas is far from "an internal issue of Israel." It is a Palestinian terror organization supported by Iran.
I won't argue that Netanyahu hasn't tried to use the existence of Hamas as a wedge between the West Bank and the Gaza Strip and a way of "proving" to the Israeli public that there's no way to negotiate peace with the Palestinians.
He's corrupt and arrogant enough to have done precisely that.
However, there's a huge distance between opportunistically using the existence of a terror organization in charge of the Gaza Strip to your political advantage and causing it.
The responsibility for that is shared between the Paleestinians and the Iranians.
As for the cost to Israel, that's already incalculable.