Complex sentence: However, he saves Prothero, Lilliman and Surridge, (the three most responsible for the experiments on him) for last, showing only the remorseful Surridge a bit of mercy by injecting her with a painless poison in her sleep.
Split into simpler sentences: However, he saves Prothero, Lilliman and Surridge (the three people most responsible for the experiments on him) for last. He shows the remorseful Surridge a hint of mercy by injecting her with a deadly but painless poison in her sleep.