Simon: Money is shit to me. I would not give up McClane for all the gold in your Fort Knox.
Simon: Said Simple Simon to the pieman going to the fair, "Give me your pies... Or I'll cave your head in."
Zeus: What the hell's all this got to do with killing McClane?
Simon: Life has its little bonuses.
Answer: I've always wondered this and I don't think you're going to find a good answer. I know everyone is saying it was a tranquilizer. But tranquilizers wear off and if one of those people they gave an injection to regained consciousness it could be a big problem for them. eg. The cops in the subway. That's why I think it was probably a fatal injection of something.