I noted you mentioned you are good at math and want to help people. Engineering would definitely address the latter, all engineering fields make a significant impact in people's lives through their work. The specific morality of such an impact (helping vs. not-helping) depends on the specific employer & your personal moral values.
Example, Biomedical companies focus their R&D on medical devices which help people with ailments. But any corporation's first responsibility is to the share-holders, where making a profit is at the utmost priority. Since most engineers work in private industry in developing or maintaining technology. They have to be okay with the duality of their jobs...make devices that ultimately help people (moral) but also make a profit (may conflict with the first). On the other hand you may work for the government, where your work maybe helping some people (moral)....but may also be used for political gain, armies, foreign influence etc (may conflict with the first). Overall any technology will have it's pros and cons, and engineers have to be okay with this duality.
The reason I talked about the moral ambiguity is because, some other career paths may not deal with this dilemma or to this extent. For example doctors or nurses help the sick....which is much more straight forward. So if helping others is your main calling, where you cannot deal with ambiguity or want to make an immediate impact, then I would propose pursuing other professions (which also require math).
Now to address your interest in math. Engineering uses mathematical principles, but is a lot more heavy on problem solving & science behind how things work. So if you like math because its straight forward, you follow a set process & obtain a correct answer. Once again engineering a lot of times, is not that straight forward. We apply a lot of simplification, conservative assumptions to get to an almost-correct answer which we can use to solve our immediate problem. On the other hand, math aims to describe the phenomenon is the most accurate way possible through a derived set of rules.
Now if you are still certain you'd like to pursue engineering. Then in-terms of choosing a major, I would suggest going for the "core" majors in your bachelors i.e. mechanical, civil, electrical or chemical. Mechanical engineers covers aspects you'd study in biomedical. But also gives you a more generalized skill set. This is important because if you are looking to work right-after bachelors, then mechanical engineers can compete with biomedical engineers for biomedical companies. But, on-top of that they have an even wider pool of potential employers. On the other hand biomedical engineers are looked-upon as being more specialized so they are limited in their employments. A lot of biomedical engineers also have to pursue a higher degree (masters or PhD) to work specially in R&D.
Hope all this helps! Feel free to reach out/comments if anything was unclear.
From my point of view, biomedical engineering is something that will probably continue growing in the coming years. With computer technologies improving every day, there are a lot of applications in medical area that will be opened. I believe biomedical is a promising and growing area and as the others said, engineering is a good general area.