If Israel was behind this, they should be condemned. It's fortunate that dozens of innocent civilians weren't killed or wounded in the blast.
I don't think Israel would do such a thing in Beirut for that very reason. It's great that the guy was killed, but there was no need to endanger others. I suspect this was the result of Palestinian infighting.
Why? As far as I'm concerned they would be totally justified taking out as many of these monsters as possible.