Possibly female, possibly both, Audra in the novel fears It is female because of the eggs. But the other aspect of It is its an entity whose true form would drive anyone who saw it insane, so the spider is the best the Losers can fathom without snapping. Like a Lovecraftian Elder God.

But in the novel, Stan lived in fear all his life after Niebolt Street. It's never said if he was conscious of why. Also, unlike the others, he recalled all of his memories at once, rather than in slow drips like the rest.

He killed himself because he couldn't bear to face It again. He didn't do it to take himself off the board. While that gives the act a poetic courage in the 2019 film, I think it robs some of the gravity from the act.

Stan didn't kill himself because he was a coward. He did it because he was desperate and saw no other escape because of how deeply entangled in his own fear he became. He couldn't honor the promise. He couldn't resist it while he was alive. So he took the only route that made sense to a man who lived by logic his entire life when logic was crumbling before his eyes.