Feeding in the cage will not lead to aggression! Monitor lizards are highly food motivated so if the only time you open the cage is to feed them, then yes the lizard will be expecting food and will act accordingly. Frequent handling and target training will help keep your monitor calmer and more friendly. You can target train with either an object or a sounds. Brian Barczyk has a lot of videos of target training with monitors if you want to see how he does it.