If you end a task that Android keeps starting back up it will increase your battery usage, not decrease it. Something sitting idle, using no cpu, will use cpu to start back up. Should you want, you can monitor the ones that keep starting back up on their own and ignore them and only "kill" the ones that don't start back on their own. In all honesty the only reason you'd need to manually kill anything is if it is acting up and using cpu when idle.
The reason it keeps things in RAM is so they'll start faster when you use them and if you continually kill them individually you may see performance lag in them.
I'd suggest an app like Watch Dog Lite, or Gsam Battery Monitor to get a truer picture of the cpu usage and battery usage. The built in task monitor is more like something you'd see on a Windows machine and is woefully incomplete for Android purposes because of Androids inherent design to keep RAM full, not empty.
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