Ken Shamrock -- Kimbo Never Got the Respect He Deserved (VIDEO)

Ken Shamrock says his heart aches for Kimbo Slice -- saying the guy made a tremendous impact in the world of combat sports ... but never got the respect he truly deserved.

Shamrock -- who fought Slice (and lost) last year -- says Slice made one helluva splash in MMA and boxing ... but still had to battle extreme negativity from haters -- even when he won his fights.

"Even when he won a fight it seemed like he was never good enough," Shamrock says.

"When you really care about your fans and you really care about what you do then those things that come at you from a negative position ... it hurts ... especially when it comes in thousands of people with negative thoughts. It stings, it hurts."

Shamrock says he never spoke with Slice directly about it -- but says it was obvious Kimbo was affected by the negativity.