It's an incredibly stupid, dangerous thing to do to throw objects into moving traffic. It should be dealt with harshly; there must be consequences.
Vigilante murder shouldn't be one of them. We live in a civilized society. We don't operate according to such vendetta justice. And potential manslaughter, horrible as it is, still doesn't equate to cold-blooded murder.
And I doubt the driver was thinking about how he might swerve and strike someone else. He was probably thinking about having to get his car cleaned.