There is, however, reason to disapprove of the decision to kill him. Here's why:
Israel had previously tried to kill Rantisi on June 10 when three Apache helicopters fired at least seven missiles toward his car in a crowded Gaza thoroughfare, reducing his vehicle to a scorched heap of metal. Rantisi escaped with a wound to the right leg. Two Palestinian bystanders were killed.
In a retaliatory attack the next day, 16 Israelis were killed in a Hamas suicide bombing in Jerusalem.
Actions have consequences. A given action (such as this one) may be laudable when considered in isolation. However, few actions are isolated... and the consequences of killing Rantisi will certainly not be borne by the Likud advisors who authorized it.
UPDATE: Of course, Israeli partisanship has no shortage of depraved, immoral ghouls. Crowing about the death of a child - any child - is a clear marker of evil.