Phone won't turn on after apps crashed one after another

Hi. I am the owner of an Android KitKat phone from a regional Romania-based company called Allview. They make cheap but fairly good phones.

Some days ago my phone suddenly started behaving odd. Sometimes, apps would crash one after another, Google services would crash repeatedly, and eventually the phone would reboot. A few times the screen image scrambled. The battery would also discharge abruptly and for no apparent reason. Factory reset did not solve anything; faults worsened and after a few days from the initial symptoms the phone refused to start up. Now the phone won't turn on whatever I try.

A friend of mine suggested that the battery might have caused that, since a faulty-working battery results in arithmetic faults in the processor. The battery does not look bloated, though. What is you opinion? Thank you.