Malaysia Airlines MH17 - Shot down by Donetsk Republic separatists

Here's the culprit. Operators already offed as part of coverup

Rebels showing off their hardware before attack

Separatists are trying to cover their tracks by getting the hardware out of Ukraine

Buk being transported back to Russia with two missiles missing

Why did the tragedy occur? system too easy to use

The missiles are straightforward to operate and work as stand-alone weapons — they can function outside of a sophisticated networked air defence system.

The system cannot tell the difference between civilian and military-type aircraft based on their transponder signatures alone. In order to tell the difference between targets, it would need to be interfaced with other weapons systems that can work off of additional information.

"When Proxy Wars go Wrong"

Lot's of blame to go round, depending on your point of view:

• Ukraine escalating conflict against separatists including airstrikes and armour attacks with tacit American support.
• Separatists operating advanced hardware without the right training (not trained to visually confirm whether plane is civilian or military)
• Russia tacitly supporting, providing expertise for operation of Buk, but not the requisite level of training.

What happens now? 

The Russian Separatists are attempting to cover it up, and have probably killed the Buk operators to hide their tracks. It may also give the Ukraine government an opportunity to intensify air attacks against separatists due to reduced air defences.

More info:
BREAKING: Russian Terrorists' Cover-Up Operations to Destroy all Links to MH17 Air Atrocity