My first guess was E2 Iris but apparently Candice filmed a lot for 2x14 and the killing happens in 2x13. It would have made some sort of sense story-wise. Just imagine E1 Barry pushing E1 Iris away because he'd seen a villain kill her E2 counterpart in front of him.
Most unlikely options: 1) they introduce E2 Cisco/Wally only for them to be fridged soon after 2) he kills Killer Frost
Most likely options : E2 Barry and E2 Joe.