I forgot all about my rainwater and it was only until recently I decided to go outside absorb some sunlight then realised that I left a Jar outside to collect some rain, and now I have six dead flies. I’m a little wary about using the rainwater.
I guess I could just take them out and ignore the fact that there was dead flies but I feel that I need a second opinion…Coming to think it maybe it would give it a boost of energy.
What did you originally want to use the rainwater for?
I was going to cleanse my room with it. I do have spring water and mineral water so I think I’ll be alright.
Since some things have died in it, and likely various things are already growing and living in it, in my opinion it is not suitable for purification. Consecrated spring water can be good!
I second what @Dankquanicus . Might be suited for curses but not for cleansing with the traces of death energy in it from the dead flies. Drowning is not an easy way to go so their dying suffering has likely tainted it with traces of negative energy imo. Better to start over with fresh water.
Quite some time ago I was enjoying a cold porter beer and when I got to the last mouthful I felt a fly with my tongue and spat out the mouthful. A fucking fly with a kamikaze death wish had drowned in the dark beer I had slowly consumed. Did I get sick? Shit no! My immune system is in top fighting order.
I’m obviously not counselling that you drink your own dead fly water, I’m just saying that you should. Do it with the attitude, ‘Fuck you! If I want to drink dead fly water what are you gunna do about it?’
Al.
He didn’t say he was gonna drink it he was thinking of using it in a cleansing ritual of some kind Al.
Add: not that it matters after all they always say “do what thou wilt be the whole of the law” (what they forget to add is and accept the consequences if and when they happen). So if he “wilt” then he “wilt” if he won’t he won’t 
Me, I’m sure we all have drank dead fly water after all we all drink processed fish toilet water (except for the people drinking evaporated fish toilet water (rain water). Not joking. All the water including rain is recycled evaporate rain pool/pond repeat. If your lucky you get treated water (or unlucky depending on your perspective).