He can wreck the happiness, or strike down the life, without, to his knowledge, any fear of suffering for it himself.

What is to prevent him, being the man he is, from going straight to his end, on those conditions?
Will the skill in rowing, the swiftness in running, the admirable capacity and endurance in other physical exercises, which he has attained, by a strenuous cultivation in this kind that has excluded any similarly strenuous cultivation in other kinds--will these physical attainments help him to win a purely moral victory over his own selfishness and his own cruelty?
They won't even help him to see that it _is_ selfishness, and that it _is_ cruelty.

The essential principle of his rowing and racing (a harmless principle enough, if you can be sure of applying it to rowing and racing only) has taught him to take every advantage of another man that his superior strength and superior cunning can suggest.
There has been nothing in his training to soften the barbarous hardness in his heart, and to enlighten the barbarous darkness in his mind.
Temptation finds this man defenseless, when temptation passes his way.
I don't care who he is, or how high he stands accidentally in the social scale--he is, to all moral intents and purposes, an Animal, and nothing more.

If my happiness stands in his way--and if he can do it with impunity to himself--he will trample down my happiness.

If my life happens to be the next obstacle he encounters--and if he can do it with impunity to himself--he will trample down my life.