You compare Kylo at the beginning of the movie to Kylo at the end of the movie and ignore every instance in between where his flaws, weaknesses and errors are progressively shown. On Jakku he acts badass, but not much later we see him struggle with the light side; he captures Rey, but fails to get the map out of her; then she even escapes, the resistance attacks and his father shows up, things get worse and worse for Kylo and in the end he has a lightsabre battle while emotionally and physically wounded. There clearly is a progression.