I would focus on the actual problem (you are experiencing high pings) rather than criticise the roommate's behaviour.
Hey ROOMMATE, I play online games at night which is super sensitive to ping, I've been noticing I've been getting very high pings at night sometimes. This has been negatively impacting my gaming experience.
Note that it's entirely possible that the internet in your area was just congested at that time and it's not your roommate's fault, it's possible they wasn't using the internet or it's possible it was them.
You seem to understand this which is already very kind and sensitive of you (good job). The important distinction is to not start with blaming them for anything especially since this sounds like it's a problem they might not even be aware of or understand.
Then, I'd contrast it with experience from your relationship as roommates.
I'd like to think of us as friends who help each other out. I value our relationship as room mates and I think it's great we're getting along. I want you to know I'm here for you.
Some people have a hard time giving compliments or being open about their feelings. If this is hard you can practice.
Then I'd ask them about solutions:
Do you have any idea how we can figure out what's causing the high pings? I think it might be the ISP or the fact you're using the computer at the same time or something else.
You can suggest that they try to use the internet at the same time as you play and measure how that impacts the pings. Make sure this isn't about them being wrong or bad or right - it's about addressing your issue as roommates who work together and want each other to have a pleasant home life.
Note it's important to be open about these things - the kind thing to your roommate is to tell them about the problem so you can figure it out together.
Then, you can figure out how to solve the issue:
- You can use a tool that does traffic shaping on their computer to limit the bandwidth he uses on your internet to a point it doesn't bother you (like, 1MBPs) on the router or their computer - maybe that's fine by you and doesn't bother them.
- You can purchase a second plan (which is typically discounted) from the ISP together and not share for the plan.
Remember, focus on the problem that's bothering you (high pings) and not the problem of fairness (he's not paying for the internet) unless that problem bothers you. If it does - it's also fine to ask them to pay for the internet but then expect them to have equal footing and have an easier time making demands about the quality of service as well - so I would not recommend it for starters.
Watch out for safety cues (like the roommate getting defensive) and if they do - step back and emphasise the shared goal (living well together and getting along) and your willingness to figure out a solution together.