i respectfully disagree with your opinion. some of tools songs may sound dark, but that is where the "depressive" feeling ends.
'stinkfist' can be interpreted in many ways, one of them being about sex. if you find sex dark and depressive, and dont listen to wierd techno while wearing black leather assless chaps, then you are the wierd one.
another one: 'lateralus' is about spiritual enlightenment. just ask enigma88. (im surprised he hasnt replied to this thread yet)
the band is also known for writing songs that joke about things so if it really seems that dark and depressive, keep in mind that it could be a satire.
theres more google "tool faq". its long but worth the read for a tool fan or for someone who simply wants to read and has the time.