It's not your business where someone else is messing up and they may be offended if you tell them, even if your intentions are good. If your intentions aren't good, you shouldn't say anything. However, if you think it's important and you can benefit the person by telling them where they're out of line, you may want to tell them. It's your choice and you know what's best in the situation. Best of luck!
Absolutely! If their lack of consideration for others posses the possibility of offending or causing harm to others. It behooves all of us to tactfully and respectfully bring to another's attention where they might readjust their words or action.
I have family members who I feel, need to be told how I feel about them. They did some horrific things to another close family member and I've never been able to get any closure for this in my heart. However, I know that if I write these people and tell them off, it only shows that I am no better than they are. You don't know how strong this feeling is to write the letters but who would it really be beneficial to? After this family member was taken advantage of and I found out about it, I immediately did something to make sure it never happens again. Those who hurt this family member know exactly what they did and from what I've found out since, none of them have a shred of remorse. No apologies, no attempts at reconciliation, nothing. At least they showed their true colors and for that, I am grateful. So, it's up to you to decide how telling someone off is going to benefit you. Just remember that once the deed is done, it can't be taken back.