The point is, like him or loathe him, mcGuinness, by agreeing to become part of the democratic process is one of the people who helped to move the peace process forward. He took the way out of terroism that avoided him being shot or jailed for life.
In this particular instance he has acted in a principled way by stepping down. It's really Arlene Foster who should have been the one resigning.
Yes Foster made a collostal error but not the same as killing people. As a politician Foster could do something to change the daft process she created. Another thought is "did she benefit personally" but that is for others.