No it's not. Rangers killed lots of folks in South and West Texas. Vaqueros and Tejanos still sing songs about "los pinches Rinches". Many of the ballads that they sing tell sad stories about what happened. It's the Indian oral tradition. Gotta go to church. Be back later.
When the first ranging companies were formed in 1823, the Anglos they were protecting from the Indians, mainly Comanche, were part of Stephen F. Austin's colonies.
These were not the common farmers who hung on at the edge of civilization. These men were all very young, with a love of adventure, and a total lack of fear. They rode their own horses and provided their own guns. They had no uniforms or badges.
They were always outnumbered, and poorly, if ever, paid for their services. They lived on what they could scavage from the countryside, including some of the farm animals they were supposed to be protecting.
When Texas won it's independence, the Rangers main duty shifted from protecting against the Comanches to protecting against marauding Mexicans who came across the Rio Grande to steal horses and cattle.
In order for the tiny bands of Rangers to fight the much larger raiding parties of Mexicans, they had to learn how to think like a Mexican. History records that more Texans were killed by Mexicans at "parleys" than during all the battles. The Rangers parleyed with their guns.
The Rangers were never cruel. They didn't torture or brutalize their captives, but they learned quickly that it was a favorite method of both the Mexicans and the Comanches if they surrendered.
The Rangers viewed the Indians in a completely different light than the Mexicans. Indians were wild wolves who had to be exterminated without prejudice. Mexicans were hated with a passion because of their duplicity.
The single most important factor which made the Rangers successful, other than the men themselves, was the invention of the revolving pistol by Samuel Colt in 1838. It was called a "six-shooter" in the West, never a revolver, and, even though it had to be broken apart in three pieces to reload while on a galloping horse, it gave each man the firepower of six.
Rangers did not respect the Rio Grande as the border between Texas and Mexico, and the Los Diablos Tejanos, Texas Devils, were feared in every border town.
These were hard men for hard times.
Their actions cannot be judged fairly one hundred sixty years later.