Evil leapers glow red when they leap. They are part of an evil organization. that does the opposite of Sam's Quantum leap project: They put things wrong that once were right. Although the amount of times Sam manages to do this himself suggests that he didn't need Evil leapers to make matters worse.
Their motive seems unclear, although they seem to be running under a separate counter-effective leap project, perhaps to try to cancel out effective changes made by Sam and Al to the quantum timeline. It's possible that they come from an alternate future timeline, where the Quantum Leap project of the future was controlled by malevolent and evil-minded people instead of good, kind-hearted people. In order for their alternate timeline to continue they must ensure bad things happen and bad people take control, to ensure the continued villainy of their organization.