Higher crime rates seem to be a feature of many towns that are dominated by the military (ex. Fayetteville, NC, a city very similar to Killeen, has a similar reputation).
There are definitely parts of Killeen you want to avoid in my opinion and target a certain minimum rent/unit depending on the property type. But it's not the case that Killeen is a giant gangland or anything. I personally live in Killeen and have never had issues. Killeen has seen a LOT of growth, and the newer housing and neighborhoods are just fine, in my opinion.
Combine that with the fact that the rental market is strong and you can still actually cash flow unlike just about anywhere else in the Texas Triangle, and it's definitely worthy of your consideration in my opinion. I've worked with quite a few Californians and New Yorkers buying $80k SFH rentals, $150k duplexes, $200k fourplexes, and $7M 160 unit apartments in our area. All of these folks are multiple buyers, suggesting they've had success and been happy with the properties they've been collecting so far.
So, bottom line, Killeen can absolutely rock it. Just know what minimum criteria you need to target. I talk more about what I look for on my own blog in the investor articles.