Why?

Ｔｈｅ Ｂａｃｋｐａｃｋ

Yamaguchi knew exactly where Suga was and what he was going to do to him...

Suga's Death

Yamaguchi trailed away from everyone with his backpack in hand. Yamaguchi had an evil smile on his face...Yamaguchi thought to himself; I'm going to kill people on this trip even if they don't deserve it...Yamaguchi saw everyone getting ready to go look for him. So he stayed behind making sure he had everything he needed to kill Suga.. he grabbed Suga while he was walking and put him into an abandoned cabin. He was going to do something similar to Oikawa's murder...Yamaguchi checked everything off..

Knife: Check

Gloves: Check

Gun: Check

Chainsaw: Check

Notebook: Check

Tape: Check

Rope: Check

After Suga's murder Yamaguchi ran back towards the group saying that he forgot his backpack. But he never did forget anything... When he saw Suga using that flashlight asking for help by using morse code.. he walked away from everyone and grabbed Suga and brought him back to the cabin.. and I think you know the rest of the story... the dead bodies that wasn't him... someone else had done it along with the deer sacrifice. 

What happened with Oikawa's death?

Well when it was late at night yamaguchi lured oikawa away from the group...and into the woods.. far from where they were staying... he killed Oikawa.. and made sure he didn't make a sound... Oikawa begged and pleaded Yamaguchi to not kill him but he simply disagreed. And he then slit his throat. Oikawa survived that so he stabbed him many times.... and then he just left him there... The trail of blood was from another person who killed an animal..