yeah..cudnt understand y the colonel kissed him..
was he gay or did he wanted to find out whether Lester was gay..??

if so it was obvious he wasnt..so y kill him..??
(American beauty)
No one can explain why he killed Lester...there can be multiple answers.this is how i figured .(Open for other opinions) may be one of the below can be the exact answer ..
1
Colonel is confused and horrified at his realization that he himself is gay and the fact that he was turned down Lester...and he was so humiliated.So he shot Lester.
2
He wanted to find out his son is gay or not.then he acted him self as gay to check it.After the denial by Lester he found out his son is not gay.but by doing so he was so humiliated and couldn't bear the shameful feeling. so he shot Lester.
3
Colonel had some serious homosexuality issues of his own.he was genuinely jealous and furious of his son, and couldn't help himself. Afterwards, the shame and humiliation he felt was enough kill Lester.