No question, the defensive player confronted with, what may well have been, a surprise choice, chose wrong and violated a rule, for which he definitely earned a penalty.
As for, "clearly uncatchable, clearly a defenseless player, clearly intent to injure", that seems like an awful lot of mind reading. Some may even favor, because of the extent of the injury, dragging the kid off the field and shooting him, but that's all based on a lot of conjecture and speculation.
As pointed out above, one of the problems with isolated film, it never shows the "tone" of the game leading up to this incident, and whether or not this had been a well played, evenly and fairly contested, well behaved competition up to this point, versus a chippy, unsportsmanlike problem, likely had some bearing on the decisions made, AS WELL THEY SHOULD.
Specific incidences are responsible for themselves, but do take place within a greater context.