“he sent away the people who had carried the tribute.” The tribute given to the king of Moab would have been quite large and required many men to carry it. Beyond that, however, it was often the custom to employ more men than really necessary to carry a tribute to royalty to give the impression that the king was really getting a lot of tribute and a tribute carried by 100 men was considered more honorable than a tribute carried by 50 men even if the tribute was the same but each individual load was smaller.
That Ehud gave the tribute bearers time and space to escape if his plan failed shows that Ehud really cared about others. He did not have a guarantee that his plot to kill Eglon would succeed; it was risky. And if it failed he did not want to have other people die with him. Killing Eglon was a one-man job and he did it alone.