As with all good English, we must review the antecedent in order to fully understand what the "it" is.
Unfortunately, this adds to the selfishness of Vladek's character. Prior to this, he says:
[As narration:] To see what kind of housekeeper she was, I peeked inside Anja's closet. [Vladek, in frame, then states] Everything is neat and straight just the way I like it! But what's this--pills?! [As narration:] I wrote down every pill. If she was sick, then what did I need it for?
Here, this shows Vladek's callousness because he means that if Anja was sick, what did he need this type of trouble for? To clarify further, he means that he was going to not marry her if she was sickly because for her to be sickly would be too much trouble for him.
On the page prior, Vladek admits that Lucia was much more attractive, but he says that "if you talked a little to [Anja], you started loving her more and more" (Spiegelman 18). In addition--on the same page--Vladek tells us that Anja's parents are also millionaires, an attractive reason as to why he may have married Anja.
Overall, that moment truly speaks volumes about Vladek's character; he would drop Anja at a moment of possible sickness because he would not want the trouble of taking care of a sick spouse.