We don't know who killed these precious girls. We have not been offered any POI's by LE or a profile for that matter. To say this HAD to have been sexually motivated and ALL child murders are ALWAYS sexually motivated is irresponsible and does a great injustice to them.
If this was retaliation or revenge, which has been discussed and is a possibility, or the girls saw something they shouldn't have, it is absolutely possible there was not a sexual element. We simply do not have enough information in this particular case to rule out potential suspects based on what one thinks is a constant. While I agree that a sick sexual element is likely, I keep an open mind as to not pass over all possibilities.
It always helps to see things in numbers, so here we go.
The studies of child abduction and murder that have the largest sample of non-family crimes, such as the Keppel and Brown study which looked at over 500 cases, have found that offenders who abduct and murder children have a primary motivation that is sexual between 70-80% of the time. Profit motivations are around 3% and emotion-based primary motivations were seen around 8% of the time. (But see my note below.) The rest of the cases had "unknown" motivation. The authors of the study pointed out, however, that in a portion of the cases where the primary motivation was not sexual, the secondary motivation was sexual. (Note: "child" in this study was anyone under the age of 18, so some of the abduction/murder cases represented peer-related aggression by other teens.)
Therefore, the children murdered in non-family abduction situations are killed for sexual reasons the vast, overwhelming majority of the time, especially when their abductor/killer is an adult. Sure, you'll be able to find a few outliers, though.