When you don't know how to eco :D

At first I thought that he just wants to pick up my spot with an early push, but after that he just started to play aggressively, it was inconvenient. A very aggressive game in ffa is always a defeat.
 I just had a lot of games when I could not destroy an opponent who was several ranks lower than me, my aggressiveness was a mistake and I just lost to a stronger opponent who could just come and destroy me.
The conclusion that I made from my past games is that you should not try to destroy one opponent and be fixated on him the whole game.

Getting bored when losing :D. Check!

I am toxic you said it yourself :)

I see

Generally in FFA you want to only take moves that make you stronger compared to everyone else. Killing enemy units for free makes everyone else weaker than you on average, so is good.

But killing an enemy unit costing 1000 by sacrificing half of its cost in your own units in a four player ffa makes every other player on average 333 metal weaker while making you 500m weaker - so you better have additional ways to profit from this trade!

This is entirely different to 1v1 where your enemy's losses are your gains, one for one.