I do think criticism can be constructive, but it has to be carefully worded. For starters, it should not be called ‘criticism’ : ) But rather just your ‘humble opinion’. I also think starting off with things like:
I know it’s easier said than done but…
I’m certainly no one to say, since I’ve made so many mistakes myself but…
I know it must be hard, and I’m not sure I’d have the strength to do the right thing but, I think ideally one should…
I think the key thing is to convey that your ‘feedback’ (for lack of a better word) is not judgment. There’s nothing worse than judgment, when one knows one is not in an ideal situation. Judgment also makes you feel criticized, and ostracized by the judger, and hence invites similar treatment (“oh you think I’m bad? Well what about YOU? how about that time when YOU did…”). It also makes a person defensive – and when someone is defensive you can’t have a logical rational conversation. No good can come of it.
It’s an interesting dilemma – I was actually going to post something similar about asking how one can avoid being judgmental when you know something is so blatantly wrong. And is having a strong belief / morals about something the same as being judgmental? I don’t think it is… but the lines can be blurry…