Amitabha will rescue any beeing wishing honestly to achieve Pure Land. Of course that a being wishing that is a being with the objective of reaching Nirvana and someone who follows Dharma. Following Dharma here means to have the desire if help other beings and try to do your best to eliminate impurities.
Amitabha will not make a difference between a good monk and a killer. What happens, i think, is that the good monk developed a quiet and peaceful mind enough to "conect" with Amitabha; in other words, the good monk removed lots of karmic obstructions. The killer, the guy that dont have a peaceful mind, will be more disturbed and have more heavy karmic that will make things more difficult to him. But if he to believes in Amitabha and have an honest desire to help others and reach Nirvana (on Pure Land), these karmics obstructions will be dimnished and he will be able to be saved by Amitabha.
You must see Amitabha as someone in a boat trying to save every drowning people around his boat. He will extend his arms to everybody. In the analogy, the good monk is someone trying who is peaceful enough to see that there is someone there (Amitabha) able to save him. The ordinary persons, the non believer, is just a drowining guy without the knowledge of the guy in the boat.