It could be a few things...
When you kill me, I return stronger
Hope -- You can kill hope for a short time, but then it usually starts up again even stronger
Dreams -- Same as hope, if your dream gets killed normally the desire to achieve it will come back even stronger.
Weeds -- at least for me, whenever I try to pull up all the weeds or plants (like 4 o'clocks, they just won't go away!), they seem to be dead and gone for a short time and then even more pop up.
Virus -- people create vaccinations and medicine to kill and combat viruses in peoples bodies and it works, but sometimes viruses use the antibodies found and they mutate and adapt to it and come back stronger in different strains.
When you die, I live on.
Your hope and dreams can live on in other people that you touch, and viruses and weeds live on without you.