How can he be guilty of a hate crime against a group he is part of

It doesn't look like a defence. If the articles I have read about him using slurs against homosexuals are brought in front of the court, and that convinces the jury that that contributed to his crimes, he'll get 5 times 1-3 years for bias-motivated crime on top of five times life in prison for murder 1.

this a smart move by the lawyer.

It will make no real difference.