Felon means a crime so feloniously must mean acting criminally. A felon can also be an outlaw. Treason is like plotting to kill the Queen or something so I guess this means he was probably a part of plotting the MoM/DD/Harry's demise when he was a DE. Malice basically means he's malicious and cold-hearted, or that his actions were. Aforethought is a posh term which I can't quite get my head around at this time of evening