If we're being especially pretentious, this is one of the last examples of a "real" film noir.
Watching this as I work through The Wire, I was surprised as to the similarities between the two. The central conflict in The Wire is between the aspirations of the various characters and the limitations imposed upon them by a variety of institutions. The story of The Wire centers on what happens when these limitations are circumvented through some sort of moral, ethical, or legal compromise. I think a similar conflict is at the center of Touch of Evil. Orsen Welle's Hank Quinlan is not above planting evidence or framing suspects. On the other hand Charlten Heston's Ramon Vargas insists on following proper procedure.
The conflict of the film arises as Vargas questions the integrity of Quinlan's investigation into a car bomb on the Mexican/American border. Eventually it is revealed that Quinlan's investigation, while rife with evidence tampering and witness badgering, has led to the capture of the man who actually did commit the crime. However, this revelation comes only after Quinlan's downfall. Furthermore, Vargas' attempts to discredit Quinlan leads to his wife being captured and drugged. Both characters are arguably attempting to do the right thing, and the film does not hesitate to show the negative effects of the compromises required to do so.