The knife could be verified through receipts, bank records, or third party witness accounts of it being his knife. You are also avoiding the fact that he would have been covered in blood.

Like I said, the drug would be in Pauls body, and they would verify this story more by going to the lab and talking to the tech who would be able to not only give them samples of the substance to cross check, but confirm whether or not Paul had taken any.

Regardless of when they contacted the police, they did. Personally I have been in a few emergency and life threatening circumstances, and though not everyone is going to react the same, I believe I am not alone in wanting to contact authorities asap. I would not be leading a possible serial killer back to my family home.

Why are you being so flippant about a bunch of murders that are easily going to be verifiable? Wouldn't raise eyebrows? Mate, murdering one person gets a big ol raised eyebrow from 99% of the population. Those murders not only leads credence to OP's story, but shows a pattern by Serge for violence and murder. That is a massive piece of evidence.

You are ignoring all the actual evidence to speculate (please never get into law enforcement), including the biggest factor that Serge has fled the scene and is not making any of the arguments you are stating. What's his story going to be exactly? "Yeah so after I saw OP murder Paul I was super stressed so went on a long camping trip to unwind but HERES WHAT REALLY HAPPENED!", that's gonna hold up in court about as well paper umbrella in a hurricane.

If Serge is smart, and wants to continue his life of knife partyin, he is long gone and not going to come back, OP is safe and definitely not going to be accused of this murder.