Zekkel was an unfortunate Excorcists initiate who failed the secondary daemon possession trial, which was practiced among this "Grey Knights ripoff" chapter. Realizing he'd be executed after his corruption is known, he prepared to fight for his life - a decision virtually unknown of amongst the Excorcists space marines.
But, Brother Zekkel was a fellow that found life and its pleasures too hard to part with in the prospect of his gruesome demise. Though amongst Torturer's warband he's not known for battle prowess, but rather his spiritual and occult side, the single fact that he managed to escape an Excorcists battle fortress and Inquisitorial pursuiters tells that the most pacifist of the renegades harbors more power than meets the eye.
So there's Zekkel waiting for his former Brothers to come and kill him, alongside an overseeing Inquisitor Lord.