I don't think they "must" be unfavorable. I just think the overwhelming majority are.
After all it's much easier to modify the genetic code to delete an organ or diminish it's functionality, than , than it is to add a new organ or add new functionality to an exising organ.
Death does take care of severe mutations and mating preferences others. But it still seems like for every positive mutation there would be Thousands and probably millions of mutations that diminish functionality in some respect.
Thus evolution processes to be considered credible must not only find be able to adding significant functionality in relatively short time-frames, but must overcome the deteriorating effects of most mutations being negative.
Evolution is most definitely considered credible -- to those who have made a study of the subject. You might take a look at this: mutations create new variation, which I found quickly, and it only hints at the information which is available on this subject.
Coming to this subject new, as you apparently are, it's easy to be unaware that all of your objections have been dealt with for generations. If your only source of information is what you glean from hanging out at creationist websites, then you are cheating yourself. Anyway, if you are interested in the subject, you ought to study it systematically. You would be amazed at how exposure to already-existing information will change your views.
I'm moving on now. Please, don't imagine it's because your objections to evolution have overwhelmed me and that you have thrown 150 years of biology into a tizzy. I just don't have the time for this thread right now. It's been good chatting with you.
And which, if the diminishment is significant, therefore get eliminated from the gene pool. It is not significant that "useless" mutations outnumber "useful" mutations if the screening process only favors the useful mutations--which it obviously does. Most mutations, by the way, are neutral; a few altered molecules in the DNA chain will usually not effect the functionality of most proteins. So creatures carry around various forms of DNA patterns for the same functions. These sometimes get embodied as recessive and dominant traits, and hang around for millenia waiting for the right environment to favor them. The picture you are trying to work with to suggest the potency of bad genes over good genes has been simplified to the point of dis-utility. DNA is far more accomodating of useful variation than you imagine it to be.