APL (Dyalog), 17.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

###With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

###More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute (⍎) that, causing the same error as above.

APL (Dyalog), 17.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute (⍎) that, causing the same error as above.

APL (Dyalog), 18.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

###With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

###More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute (⍎) that, causing the same error as above.

APL (Dyalog), 17.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

###With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

###More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute (⍎) that, causing the same error as above.

APL (Dyalog), 18.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

###With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

###More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute ⍎ that, causing the same error as above.

APL (Dyalog), 18.5 bytes

Without bonus, 20 bytes

Goodbye Cruel World!

Try it online!

Note that the code is unquoted, so APL tries to execute it but World is not defined, causing a VALUE ERROR crash with the offending line of code included in the error message.

###With bonus, 35 ÷ 2 = 17.5 bytes

'Hello World!'
Goodbye Cruel World!

Try it online!

First prints the required string, then crashes like the above program.

###More sofisticated bonus version, 35 ÷ 2 = 17.5 bytes

⍎'Goodbye Cruel',5↓⎕←'Hello World!'

Try it online!

Prints the first string, then drops the first five characters from that (5↓), then concatenates that (,) to a new prefix, and then attempts to execute (⍎) that, causing the same error as above.