A bona-fide sociopath
I have never liked Kevin Spacey as an actor. The reason why is that he always came across as creepy to me, like an android trying very hard to convince you that it's really human but missing those subtle facial ticks and intonations that a real person would have. Picture Data from Star Trek, except with none of the charisma of a Brent Spiner coming through the character. Just a void.
Years later, I couldn't figure out how an actor so completely devoid of humanity and whose mannerisms were so artificial could be seen as "acclaimed." Then I figured out watching American Beauty that what he does is imitate other actors. By imitate, I don't mean "channel" (like how Bette Davis channeled Tallulah Bankhead in All About Eve or Johnny Depp channeled Michael Jackson for Willy Wonka). I mean he just plays a character the way another actor would've played him. If you don't recognize who he's mimicking you think the brilliance is coming from him but if you do, it's obvious what he's doing.
In American Beauty, he's not acting as Lester Burnham but playing the character as a Jack Lemmon impersonator. Before everyone calls me crazy, just know that he's an accomplished mimic and in a past life (like during the Ed Sullivan era) would've been on the same level as a Rich Little or Frank Gorshin. (So, he earns my compliments in that regard. He is a talented impersonator.)
The point is that regardless of whether or not the allegations are true, I believe he's a sociopath in real life, completely lacking in sympathy or empathy for other people. It's why his performance left such a strong impression in people's minds in Se7en. He wasn't acting. He was basically playing himself.