Just a quick note. It should be "a gang of terrorists" - plural.
You are on the right track. "assassinated" is usually used with important people, especially political figures. "slaughtered" is usually used with animals, but if the crime against a human is extremely vicious and messy/bloody, you might use the verb "slaughter".
So, of those three, "murdered" is definitely the best, given no more context.
All three are likely to be considered "premeditated".