I would begin by becoming friends with this person, make him trust me. Then I would ask if I can stay at his house because mine is being reconstructed due to broken parts of the house that "just happened". Then, I'd wait til he fell asleep. I would then grab a butcher knife and stab him again and again, then burn the body and the knife to char while washing every single thing that got his blood on. Then, , i would put a cigarette next to a flammable item in the house, and set the place on fire, making it seem like he was smoking and dropped a lit cigarette on a highly flammable item. This would cause it to look like an accident as I call 911, saying there is a huge fire. I would run out and as the firemen come in, they would find his charred body and I would explain everything about the cigarette and flammable item and how he was trapped and i couldn't save him. This would make it seem like a complete accident. Plus, his body would be so charred that they wouldn't be able to notice the stab wounds, and just in case they did i would put giant broken pieces of glass next to him, insinuating that it was glass shards that caused the stabbing. And the fire would burn any fingerprints in the house as well. That, friends, is the perfect crime