Slash was a monster.

He attacked Raph's brothers, and even attacked Raph when he stopped him. Slash was out of control and needed to be stopped. 

But inside Raph knew Slash was still the little turtle that used to ride on his shoulder and listen to him rant about his family.

The others didn't seem to realise why, but Raph knew. He was only trying to help Raph, but he'd only heard about all the bad things his family had done. He though he would be better off alone.

Raph needed family, but Spike was also family. No matter what.