A young widow’s son is kidnapped by a serial killer and will only be freed if she finishes his final murders and thus giving him an alibi.

Read that one aloud and it may help! (it helps me, even if people think I'm nuts)

Great premise ... but reword in order for the logline to flow better.

Are the serial killer's victims, male or female?

Are the serial killer's victims, male or female?

I agree with the above points -- a big issue is that the focus is on the antag's goal -- maybe try dropping ...'thus giving him an alibi...'. What about:

'When a grieving widow's son is kidnapped by a sadistic killer she must meet his terms to kill his next victim if she is ever to see her son alive again.'

...that is of course if the widow is your protagonist -- hard to discern from the logline in it's current state.

Anyway, best of luck.

It sounds like the film is coming to end before it's began.