Did Endicott... spoilers
... send Ralph to kill Colbert? We know he sent his thugs to beat or kill Tibbs after the slapping incident. And Ralph hitched a ride with Colbert as Colbert was returning from an angry, confrontational meeting with Endicott.
Endicott definitely had a motive. Just because Tibbs couldn't find any evidence against him doesn't mean he was innocent. A guy like him is good at keeping his hands clean. And if Ralph was working for him, he stood a pretty good chance of beating the rap.
Historically, in the old south, wealthy plantation owners like Endicott were the law. They had the power of life and death over practically anyone, and that definitely seemed to still be Endicott's attitude.
We got a job.
What kind?
...The Forever Kind.