It looks like he is the last defender back, if he doesn't stop Kittle it seems like an easy TD for the 49ers. If he is able to pull him down right there without surrendering a bigger play he is at the ~45 ish and a 15 yard penalty would put the 49ers at the edge of field goal range, with a kicker who is not 100%. Worst cast scenario he can slow him down enough for his teammates to get over and help him out and they have another set of downs to try and stop the 49ers offense. Unfortunately for him, Kittle goes beast mode and carries him for another 20 yards.

It obviously backfired on him, but I don't think it was necessarily the wrong play at the time, it just didn't work against Kittle.