Evil leapers emit a strange glow of red when they leap, as opposed the leapers within Project Quantum Leap, who emit a bright glow of blue whenever they leap from their leaping chamber or from one mission to another.
The Evil Leapers are part of an evil organization of the same name, The Evil Leapers; their mission does exactly the opposite, oftentimes in counter or opposition to the efforts of Sam's Quantum leap project, often in opposition to the work of the Quantum Leap project; They attempt to put things wrong that once were right. Although the amount of times Dr. Beckett manages to cause situations to worsen when he or Al's in confusion or error when trying to figure out what Sam's mission is a any particular leap themselves suggests that oftentimes, they don't need any Evil leapers to make matters worse for them or their leapee.
The Evil Leapers 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. It's also highly possible that the Evil Leaper project is nothing more than an attempt to weaponize the Leaper project, most likely for financial and material gain.