Why is owain glyndwr a seen as a rebel by the English?

Owain Glyndwr was seen as a rebel mainly because he rebelled against the English rule of wales. After a dispute over the boundried of his land in gwynedd (north eastern wales) with one of his neighbours he took it to one of the courts in the area but as his neighbour was pro English whereas despite havign served in the English army in it's wars against the french king he was more neutral, either that or the neighbour bribed the court it's hard to tell with these things. Owain killed his neighbour and after that many of the disgruntled nobles within wales rallied behind him and declared him the prince of wales, driving out the English garrisons and caputuring many of the castles built by Ed 1 he moved into shrewesbury to attack England itself. The barons of England had moved against the king and had drawn up treaties offering land to Owain but for some reason he turned his army back probably thinkign it best to hold onto what he had than risk it all. After one of the longest rebellions in history lasting well over 15 years he slowly faded away never having been caught. Despite Henry V offering him numerous pardons he never gave up the fight. Shortly after this Henry Tudor took the throne of England and this was viewed as a welsh noble on the English throne, after that they never felt the need to rebel again.