Illegal for being a crackback AND being from the side.
This is a running play to the outside, during which a blocker far to the outside comes inward to cut-block a defender who is moving outward while focused on the runner. In other words, the quintessential scenario that the crackback rule was designed to cover.
Sure, this particular defender may have had a modicum of awareness, as opposed to getting absolutely blown up out of nowhere, but it's still textbook: Runner and defender moving east, blocker coming west.
Also, I have this block at the side. The defender is facing due south whereas the blocker turns ninety degrees to straddle the 50 yard line.