people are saying that Ren is such a burden or annoying or acts like he's a saint.
But look at it in the way that he is the one who has struggled the most adapting, he didn't have much talents appart from knowledge of the time period, to be useful.
He understands that killing for self defense is justified, but is uneasy with the way they cheer when killing an enemy, with celebration of death. He has accepted that's the people of that time. But when it comes to his friends, who lived in the modern era with him and were just civilians a couple months ago, being so easy with the concept of killing. And even accepting that in a situation of self defense, but it's not an easy thing to swallow. What makes him hurt the most is not that, but seeing people being happy or excited after having just killed.
And don't come with the thing that in modern times we have war as well, or the fact that Japan did even more horrendous things during WW2. These guys are civilians, young adults, of modern Japan which only has self-defense forces. They're not the soldiers, and people today can still be critical of modern wars, and civilians are not responsible for the wars that they do not take place in.
The character of Ren is being used to portray a perspective that's perfectly normal, he's not being dumb or naive, he's fucking scared and most of us would probably have issues like his in a situation like this.