I see two possibilities:  If he wanted to keep this court, he'd have to not only kill Richard, but convince them Gwen is dead as well somehow.  Then by law of the land, he is heir and takes the throne.  They hate him, but short of killing him, they can't stop him.

On the other hand, this took planning.  A zealot opponent of Richard like Count Montierro and others would have been willing to assist him and may have done so with the promise that Roland would kill the whole court and replace them with his supporters.

Both of these, of course, assume that Roland hadn't already turned any of Richard's court.  Which, a loyal member would have warned him, so either he didn't try or everyone he approached agreed to it.