Cow slaughter is an important campaigning issue for Hindutva organisations. Narendra Modi made it a key issue in his 2014 general election campaign. According to an RSS spokesperson, "Killing or smuggling a cow is equivalent to raping a Hindu girl or destroying a Hindu temple". After a Muslim man was murdered on suspicion of eating beef in September 2015, the BJP demanded legal action against his family and leniency for the killers. Various other beef-related murders are documented here.
Legislation on cattle slaughter varies from state to state: in most states beef from bulls, bullocks and buffaloes may be sold and eaten, while cow slaughter is permitted in a few states. The majority of beef in India comes from buffaloes. The Modi government has pledged to ban cow slaughter throughout India.
Beef is seen as an affordable meat for many Dalits and other minority/ marginalised communities, while beef consumption by Hindus is also common and has a long history. It is claimed that India has a large number of illegal slaughterhouses, and that cows are often illegally transported between states or to Bangladesh for slaughter. Although there is little data on the illegal trade, as the meat trade is associated with Muslims, cow slaughter provides Sangh Parivar groups with an opportunity to target Muslims.
References: Articles on cow vigilantism and beef-related violence on Narendra Modi Facts, The Wire and Communalism watch.