it was colder to me
it seems like it would be easier to kill someone who didn't mean anything to you. who didn't really give you anything and who was running with your enemy.
she was part of the setup. he realized the pussy was a trap and was for the death of him so it was an easy decision to just kill her, when her intentions was to just kill him in the 1st place.
joe changed marlo's life
he showed him the game
how to run it, how to launder his money, got him a passport, got him connects in the court house, got him the connect with all the good dope in the city, got him in line to run the co-op
like he literally set marlo up for a LONG RUN
joe wasn't trying to kill marlo, joe was too afraid of marlo to try and kill him which is why he did everything he did, to try and form a bond and to form a trust so that marlo would be on his side
after somebody showed you all that love and you do that when you don't even have to...joe was on his way to be ghost remember? joe wasn't plotting shit but retirement.
that'd be like you're trying to run a business and some old head showed you EXACTLY what you needed to be rich. he even gave you his clientele and contacts
and gave you the building the tax breaks everything and you started caking instead of struggling and you would be caking for life
he's about to retire, so you go to his house and kill him..