Sources seem to be muddled in bias.
Well yes. Welcome to the world of historical study. You will find absolutely no source, primary or secondary, on most topics in history that doesn't carry the weight of bias, and this is undoubtedly true for the Black Panther Party.
So...a positive or negative light? Why not both? Why either?
The Black Panther Party for Self-Defense was started in 1966 with the express purpose of protecting black Americans from police brutality, and grew to become a Marxist/Maoist political party dedicated promoting national self-determination for the black community. To guarantee their ability to observe the police conducting their business, and to defend themselves against state reprisals, the Panthers armed themselves. In the ensuing decade the FBI engaged in a covert counter-intelligence program to destroy the party and jail its leadership.
How exactly one views the BPP says as much about one's contemporary views on subjects like racism, police-community relations, civil and human rights, terrorism, the right to bear arms, the right to dissent, Socialism, national security, as it does about how one studies history. You simply won't find any unbiased sources, and anyone who tells you to view them positively or negatively, however justified they might be, is pushing an agenda. It's your job to piece together, consider the matter, and form your own argument.
Couple of thoughts
There is the argument that the 'radicals' provide the moderates their 'leverage'; part of the reason reform that moderates advocate happens is because the establishment sees this as a way to stem the radicals. Without the radicals, the moderates would be ignored. See page 225 - Civil Rights History from the Ground Up: Local Struggles, a National Movement edited by Emilye Crosb
Free Breakfast Program - panthers fed around 10K kids breakfast at the peak. "So in 1975 the National School Breakfast program was launched, serving free or reduced priced breakfasts to kids in need every school day. Can we argue that this was a direct result of the Black Panthers work, it's hard to say? But it is clear that the Black Panther Free Breakfast Program scared and embarrassed the state, and highlighted the gross and endemic racialized inequalities inherent in U.S. society." http://www.organizingupgrade.com/index.php/modules-menu/community-organizing/item/942-honoring-the-44th-anniversary-of-the-black-panthers-free-breakfast-program