ABANDONED. Horror fiction about a descent into madness

[Stephen]

This one's really tough to talk about without spoilers so apologies in advance! I read it in about 2014/2015 I think but it was definitely older than that. I think the cover had a shadowy figure on it, it was written by a man, and I think it was American - definitely Western! Mature content with some gorey parts. I think the title had "Monster" or something like that in it. It was pretty disturbing but has been driving me crazy to remember the title, so any help would be amazing! Plot details below.

SPOILERS




It starts off with the male protagonist, who I think works in finance or has some high-pressure corporate job like that, having affairs with married women, and gradually he becomes crazier and more violent, eventually committing murders and (finally) drowning himself at the end.

[Laura]

Maybe American Psycho by Bret Easton Ellis (sorry, I can’t link on my phone)? I don’t remember Bateman drowning himself at the end, though.

[Stephen]

Sadly nope :( But it's definitely similar!

[Kate]

Stephen ~~
Don't lose hope and stop posting. People have found books after a few years of waiting. I did. It is quite a thrill to find a book when you thought no one would know it!

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[Kris]

No response, moving to Abandoned folder.

Stephen (OP) was last active on the site in December 2019.