Question about Dr. Kelson

Dr. Kelson seems to try and take his Hippocratic oath very seriously, refusing to kill Samson even though he very well could. However, he performs assisted suicide with Isla due to her cancer.

Now I know some doctors do perform assisted suicide in real life, but in the world of 28 Years Later, I don’t know why he wouldn’t knock out Samson (or other infected) with his darts and put them out of their misery. I believe he did the right thing with Isla, but but it seems much more justified to end the life of someone like Samson. The human in him probably doesn’t like being infected, and at the deepest levels of the soul would probably like to be released. And In all the years of his post apocalyptic medical practice, if Kelson had just put Samson or any other infected out of their misery, a lot of actual humans would have probably survived. Erik and the rest of the Swedish sailors would have likely survived atleast.

I understand he tries to keep his morals in this horrible world, but at the end of the day it would do much more good to society if Kelson just got rid of Samson and other infected. His view on the Hippocratic oath just seems to be hypocritical.