Differences in meaning when the verb tense changes (headlines)

What is the difference between the examples below?

Generally, in a newspaper, news is based on past tense (things which have already happened).

Then why are headlines written in ways that says: He Kills (shouldn't it be: He Killed)?

For example:

Parliament passes bill
vs.
Parliament passed bill

Also

He kills his friend
vs.
He killed his friend

Please help.