The first sentence contains a comma splice. So does the second. I replaced the commas with semicolons because the connection between the first and second parts of the uncorrected sentences seems too close for periods. (If, however, you’re semicolon-phobic, go ahead and use periods; they’re at least better than commas.)
There’s also a typo in the first sentence; guilt has an extra s on the front.
The third sentence shouldn’t have any punctuation other than the period at the end.
No one noticed how guilty he felt; they saw only his guilt. No one saw the pain he was in; they saw only the pain he caused. No one realized he hated himself far more than any hated him.
(Self-centered, much? *rolls eyes* Why is he assuming they don’t know, and why is he assuming he knows exactly how much they hate him, to say it’s less than he hates himself? ‘You have no empathy because you don’t think my feelings are the most important thing!’ Well, boo-fucking-hoo.)
Replacing noticed in the first sentence with realized would improve clarity.