abfackeln
to torch
Colloquial for intentionally and completely burning something down, often with destructive intent.
Aus Wut drohte er damit, die alte Hütte abzufackeln.
Out of anger, he threatened to torch the old hut.

to torch / flare off
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to torch
Colloquial for intentionally and completely burning something down, often with destructive intent.
Aus Wut drohte er damit, die alte Hütte abzufackeln.
Out of anger, he threatened to torch the old hut.

'Abfackeln' (to torch / flare off) is a separable verb. In a main clause, the prefix 'ab' stands at the end. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Er fackelte aus Versehen die ganze Scheune ab.
He accidentally torched the whole barn.
The word contains 'die Fackel' (the torch). The prefix 'ab-' here reinforces the idea of complete destruction by fire. So it means to burn something down completely.
Die Demonstranten haben eine Flagge abgefackelt.
The demonstrators torched a flag.
In the meaning 'etwas niederbrennen' (to burn something down), 'abfackeln' (to torch) is a very colloquial word and often has a negative or aggressive connotation. In a neutral or formal context, other verbs would be used.
„Ich fackel dir die Bude ab!“, schrie er wütend ins Telefon.
'I'll torch your place!' he shouted angrily into the phone.
The meaning of controlled gas burning is technical language. You hear or read this term in connection with oil production, refineries, or the chemical industry.
Aus Sicherheitsgründen muss das austretende Gas kontrolliert abgefackelt werden.
For safety reasons, the escaping gas has to be flared off in a controlled way.
to extinguish / delete
'löschen' and 'abfackeln' are opposites when it comes to fire. 'abfackeln' means that something is burned down, often deliberately and destructively; the word sounds colloquial and strong. In industry, it can also mean the controlled burning of gases. 'löschen', on the other hand, means to put out a fire. So 'abfackeln' describes letting something burn or burning it down, while 'löschen' describes stopping the fire.

Die Täter wollten das alte Lagerhaus abfackeln.
The perpetrators wanted to torch the old warehouse.
Nach dem Grillen löschen wir die Glut mit einem Eimer Wasser.
After grilling, we put out the embers with a bucket of water.