Can someone please tell me weather or not its okay to kill a fly that has lost the ability to fly ..im asking this because I know that mercy killing humans are haram.. But what about insects? My intention for killing it is not to ward off any harm but to end it's suffering... Is it permissible?
You cannot mercy kill anything, Human, animals, insects. You are killing because you think it is suffering. Even if it is, you cannot mercy kill.
If you can help a suffering soul, you help, but you cannot kill because you cannot take the sight of it suffering. You should just go away or place it somewhere safe.
A prostitute who helped a dog drink water was given Heaven and a praying woman who killed a cat by letting it suffer went to hell. This shows that every life is accounted, so by killing it you are just adding your own bad deed.
Kill it only if it is harmful to you.
In principle, it is permissible to exterminate bugs and insects which are harmful to one’s life or belongings such as pests. Bugs and insects which are not harmful should not be killed. Harmless insects should be gently moved away without killing them.
.....If these insects are harmful, you should not kill them using fire, but using insecticides. The Messenger of Allaah said, There are five animals that are all Fawasiq and should be killed while not in Ihram and while in Ihram: the crow, the kite (a kind of bird), the scorpion, the mouse, and the ferocious dog.  In another narration, the Messenger of Allaah mentioned six, the sixth one being the snake. The same ruling applies to other harmful animals, such as cockroaches and ants: you may kill them, but don't kill them using fire; rather you should use insecticides. And with Allaah is the facilitation to do what is right.
When exterminating an insect or bug, one should not oppress the creature. The creature should be exterminated quickly without giving it pain. Maximum effort should be made to minimise suffering and pain. It is prohibited to give undue pain to the creation of Allah.
It is severely disliked and prohibited to burn or drown any creation of Allah.Only in the extreme case where there is no other strategy to safeguard oneself and property from the harms of a creature can one resort to burning or drowning.
 Al-Bukhari no. 1829 and Muslim no. 1198
Abdul-Azeez Bin Baz
Fatawa Islamiyah Vol. 8 Page 283