Did anyone see Nikolay at the time that Koch and Pestryakov were going upstairs at first, and is there no evidence about that ?" "Nobody did see him," Razumihin answered with vexation.

"That's the worst of it.

Even Koch and Pestryakov did not notice them on their way upstairs, though, indeed, their evidence could not have been worth much.
They said they saw the flat was open, and that there must be work going on in it, but they took no special notice and could not remember whether there actually were men at work in it." "Hm!...

So the only evidence for the defence is that they were beating one another and laughing.

That constitutes a strong presumption, but...
How do you explain the facts yourself ?" "How do I explain them?
What is there to explain?
It's clear.