I understand exactly how you feel.
At WritersWeekly.com and Booklocker.com, we're frequently recipients of manuscripts that are...well...just plain awful. It's always hard to tell someone their writing needs work. And it's really hard to tell them when you don't know them.
I have, on occasion, been entirety professional and kind in my critiques and ended up receiving a scathing email full of hatred and profanity. But, generally, most writers want to improve and are desperate for ANYONE to give them an honest critique.
The key is to provide an honest opinion, but to do so in a positive manner. For example, instead of saying, "Your character, Marge, is really whiny and readers are going to hate reading what she says..." to "Wow, your character Marge, really reminds me of my mother-in-law! What a whiner! She's hilarious, but...you know...some people might find her a bit too whiny. Maybe you might think about making her whine a bit less, and maybe her other comments can be in anger toward the other character rather than just complaints. Readers LOVE confrontation..."
I can always, ALWAYS find something to compliment about a manuscript, even if it's just a creative character name or a unique adjective that I'd have never thought to use. Sure, some people just can't write, but most who can't would love to become better writers. We're all writing to make a living and because we love to write!
So, my advice is to provide criticism and praise simultaneously. This lessens the blow but is still constructive (and honest).
Hope this helps somewhat. :)