
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.

to flare off
In industry, especially in oil and gas production, to burn off excess or unusable gases in a controlled way using a flare.
An der Raffinerie wird überschüssiges Gas aus Sicherheitsgründen abgefackelt.
At the refinery, excess gas is flared off for safety reasons.
'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 burn down / set off
Abbrennen and abfackeln can both refer to deliberately destroying something by fire. Abbrennen is more neutral, while abfackeln sounds colloquial and places greater emphasis on the ruthless, destructive act. Abbrennen fits a factual report, while abfackeln is more suitable for an informal statement.

Die Täter haben das leer stehende Haus einfach abgefackelt.
The perpetrators simply torched the vacant house.
Laut Polizei brannten die Täter das leer stehende Haus absichtlich ab.
According to the police, the perpetrators deliberately burned down the vacant house.