Death Trap — John D. MacDonald

In life, Jane Ann never had much use for a halo, but in her violent death she finally earned one. When they found a suspect, everyone relaxed except Hugh MacReedy.

Maybe he should have stayed out of it, but MacReedy owed a big debt to the patsy they were sending to the electric chair in a week. And he would have stayed out of it, if he'd known what his chances were of coming out alive ...