What happens if I kill a mosquito that is sucking blood?

Generally speaking, it doesn't matter. Mosquitoes are the most poisonous in autumn, because mosquitoes that survive in hot summer will carry a lot of bacteria, and toxins in their bodies will accumulate for a long time. If you happen to meet a very poisonous mosquito biting you, and you happen to be killed in a blood-sucking place, it is possible to spread toxins in your blood. Of course, although most mosquitoes carry bacteria, they are very small after all. People's own immunity can fight thousands of bacteria and resist the past, so they are bitten by mosquitoes.