Kitkat LMK is very effective, i deliberately tested it, but that restarting happened too even when free ram was even 200+mbs.
I reflashed same firmware by re downloading it and now no more issue. The downloaded firmware had been corrupted.
Yes, these task killers and battery savers etc ruin everything. I use only cleanmaster once every 15 days, but i disabled its ram cleaning option.
According to user manual, clearing recent apps window closes unnecessary background apps, so i use that and greenify (non root) because phone has no root method until now.