Hi Leebo,
The thing with your revision is that it brings up more questions than you can answer and I think you would be better trying to re-work it so they don’t come up. To that extent, I think the original ‘new killing spree’ worked better.
Otherwise, you’re almost there. However, this bit – ‘he captured and saw executed …… from the grave’ needs to be more succinct. The premise sounds cool – he’s chasing down someone he thought was already dead/had witnessed being executed – it’s just too wordy right now. I doubt ‘years ago’ matters and if you word it right, the reader can surmise he’d already captured him before.
And maybe put ‘homicide detective’ and we can infer, if he’s chasing down the scientist, that the man’s a murderer.
Just throwing some ideas out there – hope they help.