Tom is the worst. We want to put that out there first thing because in the book the women are so busy being suspicious of each other that they miss the true villain right in front of their faces until the very end. And that villain is Tom.
Tom is selfish, manipulative, and worst of all, whiny. He's a pathological liar, and probably a sociopath. He's a master of misdirection. He makes Rachel feel like the dissolution of their marriage was all her fault, he makes Rachel look like a crazy stalker to the police (with Anna's help), and he drives both Rachel and Anna to crazy lengths as they begin to (rightfully) mistrust him.
Probably the best example of how Tom misdirects people is in the way Rachel feels about Megan early on, when she still thinks of her as "Jess" and sees her kissing another man:
I have never understood how people can blithely disregard the damage they do by following their hearts. Who was it said that following your heart is a good thing? It is pure egotism, a selfishness to conquer all. Hatred floods me. If I saw that woman now, if I saw Jess, I would spit in her face. I would scratch her eyes out. (3.11)
Almost everything in that paragraph could be better applied to Tom. Like, way better. And yet it isn't until much (much) later that Rachel recognizes Tom for who he is—a masterfully manipulative murderer.
The cherry on top of Tom's sundae of evil is his whininess. He has the gall to say that digging with his bare hands "was painful" (36.25)—he broke a nail. But why was Tom digging? Oh, he was digging a grave for the woman he just killed. Seriously Tom, shut up.
Tom is a terrible human being with no positive qualities but an amazing ability to lie and fool people. Did we miss something?