Could anyone tell me any major negatives with bringing Halo to PC? So far all I can think of is cracked versions and hackers (which can easily be solved, other companies have been able to do that)

I don't understand why people are even against Halo on the PC. You can build a PC for the same price of the console and it'd still work better. And you can still use a controller if you now desire.
Halo wasn't on console first, it started on both console and PC.
Because Microsoft don't give a -Yoink- what people want, they just want to sell their -Yoink- console and they do that by making Halo an Xbox exclusive.
There aren't any, most people who claim there are usually are saying it because deep down they want it to be console exclusive. (Which to be honest, isn't even a problem. I want it Xbox exclusive too.)

Oh and you're wrong, it started on Mac. Last time I checked Macs are very different to PC's.

The only reason it came to Xbox was because Microsoft bought Bungie and then changed that detail.

Basically I want it to stay on Xbox because that's where the majority of it's life has been and should continue to be. If you want to play Halo on your PC, feel free to do it when the Xbox One is streamable on PC with Windows 10. 343 already said it isn't going to happen so I don't see why people can't just move on from that.
If you want to play halo buy the console.

you don't see me begging to have ps4 games brought over to Xbox. Let exclusives be exclusive.
Halo CE and 2 are already on PC.

Now for real negatives:

Lack of return on investment. This stems from less interest in the population, increased competition, low sales of previous ports.

Taking developers away from working on the Xbox games.

Making halo not an Xbox exclusive. This decreases Xbox sales.

Probably more but I don't feel like discussing it. In summary it doesn't make financial sense for Microsoft and we benefit because 343 only focuses on the Xbox.
There aren't too many. If it works, people will buy it.