Is the practice of genetically modifying human cells ethical? Why or why not?

It totally depends upon whether modification is being done in somatic cells or germ cells. Somatic cells modification is ethically accepted because it doesn't pass from one generation to another generation but germline modification is considered as unethical because the modification will pass on to the next generation leading to the persistence of modification in future generations. The problem with genetic modifications is that the impacts of modifications are unpredictable, rather than being fruitful they may lead to lethal mutations so if it occurs in just somatic cells, then even if it is lethal/harmful, it will be confined to only that individual but if a lethal mutation occurs in germ cells then it will pass on to the subsequent generations and it will persist in all future generations.