"... since getting him we've lost three rasbora harlequin one green neon tetra and a mickey mouse platy!"
Are there any clues about *why* they died? Were they injured? Bite marks? Flesh missing? Torn or nipped fins?
"Is he being agressive and killing my fish?"
When you watch your fish, do you see your betta being agressive? Do you see your betta chasing fish or nipping fish? Do you see your betta flaring at other fish?
"Tank is a 40 ltr" 40 liters (about 10 US gal) may be a bit small for a betta along with other fish. People who successfully house a betta with other fish often have a 30 , 45, or 55 gal.
"Was he a bad idea ? Opinions would b helpful !!"
You have four dead fish. So, *if* the betta killed them, then putting him in the tank probably wasn't a good idea.
In my opinion, a betta in a community tank (with other fish) is a bad idea. A betta in a community tank that is only 40 liters/10 gal is an even worse idea. Putting a betta in a tank with other fish often results in chasing, nipped fins (that can lead to disease), stressed fish (that can lead to disease), and dead fish. Some people get lucky and have bettas that are not too agressive with other fish in a large tank big enough for the betta to have plenty of space for its own territory. In my opinion, each betta has a different personality and temperment, but most bettas are not a good choice to live with other fish. In my opinion, most bettas are better off living alone.
Information on potential tankmates for bettas list very few potential candidates, and those are listed as "maybe" or "depends"