Cats do not "grow feral", they become "stray". Those stray abandonned cats are sociable to humans and can be rehomed following an adaptation period; only feral kittens found after they are 7/8 weeks old or adult feral cats who have never lived with people are adults are true "ferals". Those cats should be managed through TNR programs. There is no "need" to kill them.
Responsible ownership + TNR is the way to handle a succesfull cat population management in 2018.