When Future Trunks traveled back in time to warn the Z fighters about two killer androids that would decimate the Earth, he was referring to Androids 17 and 18. His entire life had been dedicated to surviving their reign of terror and figuring out how to kill them. However, those same Androids turned out to be very different in the present timeline -- both in personality and in their power levels.
When Krillin later asks Trunks if their foes are truly evil beings, Trunks angrily responds that yes, they're heartless killing machines. It's a natural response. After all, Trunks has only ever known death and destruction in his timeline, caused by them. However, the present-day 18 eventually marries Krillin and starts a family while 17 teams up with the Z fighters in Dragon Ball Super after starting a family of his own. But a closer look at 17 and 18's actions during the Android and Cell Sagas of DBZ suggests they were never antagonists in the first place.
Androids 17 & 18 Only Wanted to Escape Dr. Gero
Sometime when 17 and 18 were normal human teenagers, they were kidnapped by Dr. Gero. He was looking for subjects to experiment on and apparently decided the twin siblings were ideal for his plans. When Android 17 famously murders Gero via decapitation, he fulfills Future Trunks' prediction and seemingly confirms they are as dangerous and destructive as in the future timeline.
But 17's choice was completely justified. He and 18 were taken from their family and forcibly modified into androids, then were shut down and locked away any time they disobeyed or (presumably) tried to escape. If the siblings ever wanted to truly be free again, they had little choice but to take their kidnapper's life. But from the Z fighters' point of view, they saw two new androids kill their creator and formed the conclusion that their own lives -- as well as the Earth -- were in immediate danger.
17 & 18 Are Only Violent When Antagonized
After Gero's death, Trunks fires a powerful blast into the cave hoping to destroy the remaining androids. A natural villain -- or even human -- reaction would be to defend oneself by fighting back. But 17 and 18 don't respond at all, simply waking up 16 and flying off together. The Z fighters are left confused by their actions but Vegeta, angry at being ignored, goes after the androids to fight them himself. After repeatedly insulting them and demanding a battle, 18 finally tires of his arrogance and agrees to fight.
Once again, when the other Z fighters arrive all they see is Vegeta heroically battling an all-powerful android who will destroy the Earth if left unchecked. At this juncture, 17 forms an agreement with the others: he'll stay out of the fight as long as they do. But when Trunks rushes in to aid the defeated Vegeta, he breaks their deal and essentially forces 17 to step in. The end result leaves the Z fighters thoroughly battered -- but alive. 17 and 18 could've easily killed Vegeta, Trunks, Piccolo, Tien and Krillin right then and there, but they didn't. As 17 explains to Krillin, their only real goal is to find Goku so 16 can fight him. There are no ominous plans to terrorize the human race or decimate the planet.
When the androids' search for Goku leads them to Master Roshi's house, Goku has already recovered from the heart virus and left to go train in the Hyperbolic Time Chamber. So the androids once again come face to face with Piccolo and Tien. 17 -- and 16 due to his programming -- are only interested in fighting Goku. Once again, it's the Z fighter who insists on fighting. 17 gives Piccolo multiple chances to avoid fighting if he'll reveal where Goku is. But despite knowing Cell was coming, and Kami's belief that the androids weren't a threat, Piccolo jumps into battle.
Aside from killing Dr. Gero, violence was never the first option for the present-day androids. It was only when someone got in their way and directly threatened their newfound freedom -- like Vegeta, Piccolo and finally Cell -- that they chose to fight. Androids 17 and 18 were never antagonists, just extremely powerful juvenile delinquents. And like the other Z fighters, they eventually grew up, started families and fought to protect their loved ones.
