I don’t think they’re evil per se, but consider the scope of this battle from their perspective. This is a war for the fate of the galaxy, and naturally to the Titans, their goals are the most noble and beneficial to all of creation.
Look what they did to Azeroth. They stopped the madness of the Void, and we know that life under an Old God is a living nightmare of blood sacrifice, madness and relentless change, and then resculpted the planet into an Earth-like situation, likely returning it to its former state before the Old Gods came, re-seeded it with life, helped the life-forms that survived the Old God occupation adapt to the new status quo, created layer upon layer of defences to protect the World Soul from outside forces and asked for precious little in return.
But …
At the end of the day, all life on Azeroth is effectively the decorations of a baby’s room, we’re here to provide companionship and a safe environment for that slumbering World Soul, and the Curse of Flesh got involved. All the Dwarves, Humans and Gnomes are accidents that were permitted to continue because they were deemed to be both suitable under the Titanic guidelines for organic life and useful as potential servitors for the great work of nurturing and protecting the World Soul. Elves are just mutant Trolls and don’t fall into that category, all of Troll-kind are just dubbed ‘native species’ and so long as they don’t mess with the World Soul or invoke the Void, I don’t think the Titans care. Same for the Pandaren, Tauren, Goblins and Vulpera.
Orcs, Draenei, Ren’dorei, Forsaken and Lightforged Draenei would probably be viewed as quite distressing, being foreign additions to the mix, or infused with dangerous amounts of rival Primal Powers, namely the Light, the Void and the Fel.
At the end of the day, the Titans might like us, but they’re shoulder-deep in a cosmic war that has been raging for billions of years and this is the one battleground on which they have even a toehold left, as we’ve seen in the Time Rifts, where almost every attempt by the Titans has either failed or been thwarted from within by their own followers. We need look only at Odyn to see how that plays out, where the gigantic platinum clod ripped a whole wing out of Ulduar and potentially allowed Yogg’Saron to corrupt Loken, kick off the Curse of Flesh and reduce the Titan Watchers and Keepers of Ulduar to mind-controlled puppets.
The Titans are one of the few Primal Powers who allow free will and expression of emotion outside pure zealotry (Light), madness and fear (Void) or unending rage and hunger (Fel). But because of that, their grip on the war is tenuous and if they lose this Azeroth, there’s a high possibility they may not have any other options. Re-Origination is their last move, because a World Soul consumed by the Void, or the Fel, is such a cosmic-level disaster that they’d rather wipe the planet clean of life and possibly scar, if not kill, the World Soul that allow it to transpire. There’s even dozens of checks in place to ensure Re-Origination doesn’t take place unless there is no hope at all to avert full corruption of the World Soul.
They’re not evil … but do you think about the Ants’ feelings when you spray them to keep them out of your house?