I really don't agree that killing Ren is a bad idea. I love Ren, one of my favorite characters but i don't think that this is a bad idea for the plot.
In my opinion is a good idea because this way Ryo will probably understand the last words of his father, some people only learn lessons in life in the hard way so maybe for Ryo will be this way.
Ryo will mature a lot in my opinion not only as a martial artist but as a character as well and maybe this will be in the hard way.
I imagine Ren dying maybe saving Ryo and then Ryo remember his father's words about friends and Xiuying's words about how revenge is a path of evil, then probably at this moment Ryo will feel a lot of pain and will finaly understand and mature about this things.
If this happen it will be really sad and emotional, depending on how it's done maybe it will be one of the most impactful and emotional scenes of the series.
Iwao: Keep friends, those you love, close to you..
Xiuying: You still don't understand. Martial arts are used to protect people's lives. If you use your fists otherwise, you will walk on the path of evil, too!
Ryo still don't understand the full meaning of what Iwao said, Ryo still don't understand the full meaning of Xiuying's words about revenge, Ryo didn't fully learn this lessons in my opinion, probably something will happen that will change Ryo's mind and he will even forget about revenge. Ryo will learn in the easy way or the hard way? i don't know, If Ren die it will probably have a deep meaning for the plot, something really impactful.
Right now in the story Ryo only want revenge, this is his focus right now, his focus right now is not on friendship and love, nothing like that, Ryo focus right now is only about revenge and this can destroy him, i think that something will happen for Ryo to finally understand what Iwao said and what Xiuying said, maybe it will be in the hard way, i don't know how this will be but this is how i feel about this.