A flagship is a new desirable product at the top of its niche, and fetching a high price to match. A flagship killer is a good product that aims to provide the same desirable features but at a more affordable price. If it achieves this aim, then it's a "killer" because it will have defied all the claims to top notch status.
In fact such "killers" will have made small sacrifices to ratchet down their buy price - they won't be pure gold, so to speak - but many if not most customers will go for what's cheapest provided they get the newest features. An example in 2018 is the Google Pixel 3 (flagship) and the One Plus 6 (flagship killer). The One Plus 6 has most of the features but not the truly excellent camera. It is £300 cheaper than the Google Pixel 3.
A flagship killer is a term that is widely used in the smartphone industry. A flagship product is one that is usually touted as the best product that a company produces. A good example is the iPhone X. At the moment, the iPhone X is the flagship product of Apple and its smartphone sector. A flagship killer is a product that is developed that is made to closely emulate the flagship product of another company. The flagship killer of the iPhone X would be the Samsung Galaxy 8. The Galaxy 8 was made in direct competition to the iPhone X. It has the similar specs and look but the thing is that the Samsung Galaxy 8 costs lest. That is the main goal of a flagship killer product. The product is sold cheaper but at the same time itlooks, feels, and is able to produce like a product that is widely used and popular. The flagship killer concept has been in the smartphone industry for a long time. It fosters competition but it also opens the door for cheaper materials which could be looked at as not up to par or not as high quality as some of the major flagship products.