How can I make amends with the one I wronged?
The Kamma has been laid down.

Now there's no use in scruples over the past.

The best and the most constructive thing to do I think is make a commitment to never repeat acts of callousness and actually fulfil it.

It is possible to mitigate acquired new bad Kamma, counterbalance it as it were, by development of wholesome qualities such as good will, cheerfulness and compassion.