Since beta-particles are just free-moving electrons, shouldn't they be caught by strongly-electronegative atoms and thus create anions? Why do they instead create more cations by knocking other electrons out of their orbits? Does it, perhaps, depend on the speed of electron: i.e., slow-moving, non-ionizing electrons are more likely to be caught by atoms?
Beta particle energies are of order $10^5$ eV, which is vastly greater than typical electron affinities of order $1$ eV. The beta electron is not going to end up bound to another atom until it's had a chance to dissipate the unwanted $99,999$ eV. The energy is typically dissipated by colliding with other atoms and ionising them.