Well i don't know that was so cruel but the good thing is that Kenshin only die in the ova the manga has a diferent end...

i cant believe his son though

At OVA, Kenshin die beacause he has an illness that cannot be cured, right? And at least he died at Kaoru lap, than not seeing her again...

In the OVA, Kenshin's body was consumed by an un-named disease that robbed him of his strength. It was either a rebound effect of accumulated injuries and the harshness of HMR on his tiny body, or a physical manifestation of his self-loathing. Either interpretation is valid.