Was the hit late? Absolutely! Do I think it was done to intentionally hurt DJax? I do not.
Much like how Baseball has a set of unwritten rules, football does as well. These me make their livelihood playing a sport that is brutal, not to mention has a very small window to play in. While they are out there, yes, they want to make the big hit, but they are not trying to intentionally hurt each other. Only in very rare cases, like Myles Garrett using Mason Rudolph in a VERY heated situation, would I assume a player is trying to injure another player.
Back to the Harper hit. Here is how I see it: Kick returns move VERY fast for the players on the field. Much faster for them than for you or I watching it on TV. I think that in the moment he was still in full tackle mode before he had a chance to process the fact the play was over. Watch it one more time. You will notice the ref does not blow the whistle until the moment Harper made the hit. If the whistle was blown when the first hit on Desean was made, Harper most likely would have pulled up.
I want to give Madre Harper the benefit of the doubt. Because if he was trying to injure a player who JUST came back, he would be a marked man.