How to write a negative review for a conference paper?
I think this is too much of a duplicate of the other question, but my comment got too long.


Firstly, the review process is blind, so you simply have to do the best that you can. Be objective, be polite, give good reasons – see other question. 
There is probably no such thing has having technical comments/improvements that are too long for the standard review, unless you will end up rewriting the paper. Authors tend to appreciate comments, even if they mean more work. If the paper is borderline and is worth improving, then give as many comments as you have time for.
If you have additional comments that you want to send to the authors, consult with the chair/editor before contacting the authors. Perhaps your comments can be sent anonymously via the chair/editor. 
Papers get rejected, we learn to live with it. The best you can do is provide good comments to help the authors improve the paper for next time round. Authors may get mad, but they don't get even.
If the paper is terrible, just save your time and don't bother with all the comments. Write a sufficiently convincing review giving reasons why the paper should be rejected, point out that there are many typos and other mistakes, but don't enumerate them all.
On the other hand, if you are presenting and proving an alternative axiomatisation of their results to the degree that you are not only rewriting their paper, but lifting their results to a whole new level, then you should consult the chair/editor to see what to do. Maybe you should get credit for your contribution. You certainly should never write up your results independently, not until the paper has appeared in print.