Decimate: 1/10th destroyed or killed. This word came from the Roman military punishment of killing off 10% of a military unit for bad behavior or performance. Decimation is bad.
Destroy and/or Obliterate: decimate plus the other 90%. If something is destroyed it is broken beyond repair or injured byeond recovery. If something is obliterated, all traces of it are gone.
English user pro tip: DEStroy and obliterATE do NOT combine into the compound word "Decimate." You are a jerk if you use decimate when you mean destroy or obliterate. Sad but true.