That's not a good analogy. Little Tommys generally don't belong in the road, and you would have no reason to push him there. Football is a contact sport however, and professional fouls are accommodated. Thus a collision with another player in the course of a professional foul is par for the course.
I know this is football forum and not an advanced university level seminar but surely you guys can see the principle of endangering someone else and where the blame would lie in case of potential injury?
Obviously in reality pushing someone in front of a train isn't the same as kicking someone on the field but the principle of doing something to another individual which risks injury is the same.
Since you obviously need extremely similar comparative examples to comprehend (which by the way defeat their entire point) we can just skip the similes.
It's just simple causality. Son was elbowed in the face by Gomes (unintentionally or not is irrelevant) which made him retaliate by going into a late tackle with no intention to get the ball. This action alone is responsible for the injury Gomes suffered regardless of factors like luck and Aurier being in the way because it was premeditated, intentional and CLEARLY, because we have the privilege of hindsight, endangering his safety.
Now whether or not a tackle like this should always be so harshly punished as with a sending off is debatable but he was, undeniably, making an action which put Gomes in danger, which is evident by you know.. A total fracture and dislocation of the ankle which might very well have ended his career.
Feel sorry for Son all you want, and obviously he didn't want to end his career, but it was 100 percent his action that caused it and it is always a risk when you go in for a tackle with zero intention of getting the ball. I'm sure he is distraught and remorseful but that's because he knows he should not have done what he did rather than simply struck by lightning.