Throughout the movie, Bell's character has been shown to be of a man who is surprised and sad to see the violence that humans can resort to now, sth that he's seen quite a lot in his job as a Sherif. Inspite of this being his job, Bell seems to be less determined to catch Chigurh but more determined to save an innocent (and comparatively far less of an evil) man i.e. Moss.
The reason Bell went in that room was to revisit that place again and, probably for the last time, where that cold-blooded murder took place so that he can reassure himself of this bitter and undesirable truth that humans can be the most violent and heartless of all the animals alive.
It's like, when people are faced with sth unbelievable (sth that they think could only be possible in dreams), they think of ways to prove themselves the reality of those unbelievable things in order to come to terms with the reality and by visiting that room once again, Bell was also doing the same thing. The police tape, the blooded carpet and the opened vent were all proofs that that hideous act was a truth.